Disengagement is what you make off it. Seeing your SS is still a baby basically no need to be so drastic like with older children.

Just a couple of things you can do,

1. Once a week, you need a night for adults only, nice dinner even if it's take away, steamy sex, no phones, no talking about children... it's your night, SS can be with his grand parents, heck Dad would also like a night off dammit... this will keep your relationship balanced a bit.

2. If SO asks for help, depending on what, you decide yes or no, quickly supervise the snot for an hour, no problem, bathing the snot - huge problem, you do not go there, it's his parents responsibility.

3. You are at a family gathering, SS comes to you and asks if he can have xxxx, you smile and say yes sure kiddo let's go and get you some...
SS comes to you and says he needs the potty.... no problem take his hand and take him to do what ever, then stand in the door with the door a bit closed and he can call you when he's done... you are there but not touching him unless he needs help with his trousers.
(Remember he's 4, by the time he needs to go he will not have time looking for Daddy)

4. SO wants to go out with his buddies and you have to babysit, you have 2 options, say NO or tell him sure darling, pay up front and charge babysitting fees.(but I would rather go out with my friends then..)

5. Sitting together for a meal and SS is misbehaving, keep quiet and give SO the bitch look so he can lift his ass and deal with it.

6. When with SO's family and they all spoil the child, not your problem, if they ask you how do you like SS, smile and say, he's cute, well behaved, naughty little bugger... anything that's not really saying something.

7. Times you are looking after SS, why not engage with him like his pre school teacher would, hey kiddo let's build some puzzels, color some pictures for Dad and GRan or what ever, simply sit together and do something small.... do not tell him what to do, if he says no, then simply pick up a puzzel and start...

Think about it this way, Your neighbor has a hot date with her husband and they have a kid but no other family, now the neighbor is your friend, and she asks you to babysit... (this can be your sister, brother) how would you treat that child, ... now you treat SS exactly the same way

Edit to add: When you make something to eat, like dinner... ask SS if he would like to be your little chef, he can stir , and after wards you both clean, even if he make a bigger mess wiping the table, tell him thank you he's so sweet and a very good chef... (then do it over when he's asleep)

Like she said, disengagement means different things depending on the bioparent and the age. The key to disengagement is to avoid getting stuck with any skid responsibility. You need to be upfront with your partner now before Full custody and let him know he needs to be prepared for his workload and make it clear you will not be the skid’s stand-in Mommy. When disengaging at this age, it really means staying away from disciplining the kid. Send him to his room until his dad gets home instead. Don’t become responsible for transportation, but it’s okay to occasionally offer help. When the kid has a ball game, you can go or not depending on your own plans. Tell your SO he is either going to have to start cleaning up after the kid more or hire a maid because you won’t be picking up the toys. During holidays, don’t take over and buy all the gifts. Instead you offer to watch the kid for a few hours while he takes care of Christmas. Another big one is when having a conversation with your SO about his kid, you say noncommittal such as “oh?” “Whatever you say” “it’s up to you” etc. putting all decision making on him. Same goes for conversations about BM. Convos with the kid will be easier, “go ask your dad” or “what would your dad say?” “Wait until your dad gets here”. You don’t ignore the kid; that’s for skids who have morphed into ungrateful disrespectful teenagers who know better. At four, he still learning how to treat people and that his words and actions have affect on the people around him.

Oh I thought of a couple things more....

You and SO are talking, SS comes in between, smile and say, SS adults are talking wait your turn, finish what you where saying, SO replies and then you turn to SS and say... What did you want to say sweety... if he has a tantrum and not grasping the WAIT... SO should send him to his room for time out, but again this is SO's responsibility not yours.

Sitting on the coach and SS tries to get between you and SO, grab the little bugger, tickle him and say, hey Mr Rude, I'm sitting here, but you can sit on my lap for a while....

He's really just a 4 year old, probably having accidents in his bed now and again, you have the ability to teach him now not to dislike you or be a rude teenager, now don't get me wrong... when he's a teen he will dislike you, and his father, and all off his teachers, that's simply his job then lol, but he will still come running to you if he's in trouble , then they turn into babies again..... your future with this child depends on what you are doing now, before he's 7-8.....

I think the biggest problem is your SO, he's trying to shift the parental role to you, you are not SS parent, and this you have to stop.....
Tell SO, HOn I'm not his mother, I never will be regardless of what you want it to be, I was not the egg donor. I will help you and I'm simply a nice Aunt in your kid's life, nothing more.

Unless this 4 year old is a complete monster, there is no reason to ignore him. That's not disengaging, that's cruel. You need to be clear with your SO on what you expect from him and what he can expect from you. Leave any and all parenting (discipline, school, doctors, etc.) to dad. If his family tries to meddle, tell them to butt out.

In our home I am simply SO's support person.
He makes all decisions when it comes to skids. (then there is never a question as to why or how I handled something because both skids and HCBM love to toss blame my way).
Fortunately SO does most everything. For instance, I cook but he is in charge of making sure skids actually eat. I do laundry but he is in charge of having skids put it away. He handles showers. If we are out he handles trips to the bathroom. When he is home he is the go to person. And he handles discipline! 100%.

But we are still a family. Which means when he needs me to be home (if he is working) then I am there. I may also run skids here and there. Or help with a simple project. I do skids hair and shop for clothes. Basic.

My role is more of an Aunt type figure. Which means at any time I can get up and go do my own thing

It has actually been better for our relationship. There is never anything to question because it isn't my responsibility.

As far as talking about skids - this was a huge deal for us. We found we spent a lot of our skid free time talking about and dealing with the fallout. Which turned into an argument and put us on a vicious cycle. Now SO handles nearly everything. He will ask me my opinion and I give it then we move on. He will either take it or not. The only time I will discuss anything in detail is if it affects me or my bios directly. And that is regarding everything. From HCBM to court to all the bullshit associated with that side of our life. I want no part of it. BUT there are times that because I don't handle any of that I want to rip my hair out LOL! That is what venting here is for

As far as spending alone time together - it is the simple things. We talk while each of us showers every night. No interruptions so its the perfect time. We will walk the dogs. Or just watch a movie or tv show cuddled on the couch when kids go to bed. We also eat dinner together every night just the 2 of us. After kids are done they are off to their room to amuse themselves while we eat. We try to make sure we have time to ourselves every day. And it solidifies our relationship and role with the skids. That although they are important, our relationship is always number 1.

I think it depends on WHY you are disengaging and who you are trying save yourself from.

There is teaching and/or providing your Step Children with natural consequences of their actions. But full on disengagement only works with children who are older and who can AGE APPROPRIATELY understand that:

being rude to the person who does the weekly grocery shopping = not getting your favorite snacks.
being refusing to eat the food the person who routinely cooks = never getting your favorite meals into the rotation AND not getting anything else provided for you
not putting ones clothes in the laundry hamper or away in drawers = not having the laundry person do your laundry for you anymore.
continually forgetting school/sports/work items and then being demanding/rude to the SP = the stepparent not rushing in to bring you your forgotten stuff.
refusing to talk to an adult in your house = not having that adult talk to you

And the only way for these to be effective IS to break it down by each little action and to let the child(ren) know that from this point on, you are not going to do A because you did 1 to me.

I had to do that with my SS. He would continually forget his crap, continually eat through an entire family size bag of chips in one setting, not premptively put grocery/sundry items on the shopping list and then have full on temper tantrums AT me because I would not immediately jump to fix his issues.

I could not punish SS for his temper tantrums, but I COULD remove myself from being his focus (and teach him a lesson along the way) by disengaging from each individual action as it occurred. And by letting him KNOW that from this point on, bringing his crap, buying his treats or replacing his shampoo was on his father and not to bother me. Given that his father was TDY a lot and/or worked 5a-7p, Daddy wasn't always able to fix his problems immediately either. And after about a year of adding on issues here and there, he actually LEARNED his lesson and became a more proactive and productive child.

BTW - I do this with my own Biological child. She took her Book Fair Envelope out of her backpack and left it in DH's car. He is in a meeting and I am not going to run over there and FIX a problem that she was warned about and created herself.

HOWEVER - IF you are trying to FIX your DH and how he parents and not as much about your step child(ren)s actions, then stick to just the actions of physically caring for the child, but DO NOT IGNORE A SMALL CHILD.

Seriously, the child is 4. IGNORING the child is petty and honestly counterproductive. If you have a hard relationship with this pre-schooler now, what kind of relationship do you think you are going to have when he is 10 and you have been ignoring his presence for 6 years? Why would HE respect you if YOU couldn't be bothered to acknowledge his existence?

If your DH will not allow you be an equal partner in parenting a child living under your roof, to the point you have to ignore a 4 year old, then I would leave the marriage. Because that is an indicator of how your DH thinks/feels about you

The kid is 4. Who going to take care of him ? Who working ? What your hours ? SO. hours ?
You can’t be left alone is a house with 4 YO and disengage !!! He will be glued to you.

SS is delayed or more in basic skills. He is going to need lots of attention and being worked with. He could be disabled and it will not get any better. He will always need lots of attention

You really don’t know how much this is going to change your life. No more running around with out anything on. No more sitting with SO with a third person. No more just doing what you want. Breakfast lunch and dinner.

First figure out how he is going to be taken care of. Each hour everyday Where he is going to be and who taking care of him and it includes Saturday and sundays and Holidays

Who going to be emergency person for SS. SO works late, gets the flu, has to do something

Must build in a data night every week. Only for two. And some vacation trips only for two even if only two or three days

Your not going to have an relationship with SO, yes you are going to have to move out. months. More like 8 when you start looking and packing. Try to do things with friends so you are not home

I don’t know the story but SO. Is putting DS and there relationship overnyou

SD20 was 14 when she moved in with us. I "disengaged" by not doing anything for her. I did not do her laundry, I did not cook for her, I did not taxi her, I did not pick up after her.. I left ALL of those for DH to do. He was the one who wanted her to live with us, NOT me. He is her parent, NOT me. DH and I also went around about the whole "parenting" thing. DH thought I should "act like a mother" during convenient times such as by giving SD rides, by helping SD with homework, by putting SD on my cell plan, by asking her how her day went etc. But on the other hand, DH thought I should "mind my own business" and let HIM parent (or NOT parent) when it came to things like SD failing school, SDs phone bill being $700.00 one month, SD drinking, SD being a huge slob etc. I informed him that he couldn't have it both ways. It's all or nothing, so it was nothing.

Its hard...ive been on off disengaging for years!

Fulltime no easy way, unless you leave house or have them go out, or hide out for a bit.

Dh will resent you.

You are ignoring a piece of him.

But as the other ladies say, its different for everyone.