Newly blended, big age gap.
My husband and I have been married for almost a year, without going into a crazy amount of detail, his 2 and 4-year-old started living with us during the week about 7 months ago, my 11 and 14-year-old started staying with us on the weekends and sometimes the week during remote learning (they live in another school district) about 3 months ago. It has been ROUGH when they are all here together, or even when just my kids are here. My husband is very withdrawn with my kids and seems to look for stuff they are doing wrong, I feel, while seemingly ignoring the bad behaviors his kids have. I think the age difference is an issue as well as it just being new. Sometimes he holds my kids to a higher standard because "they're older, and should know better" but sometimes he expects them to do exactly what his kids can do, and no more, because "the rules apply to everyone". It is seriously straining our marriage at this point. Don't get me wrong, I do NOT think my kids are perfect and they do need to be disciplined at times, which I do, other times it just seems like ridiculous small things that shouldn't be a big deal. My husband seems not to see how terribly rude and disrespectful his 4-year-old is at times and goes un-punished, but if my 11-year-old says anything, to his kids if they are doing something wrong, he tells her to stop telling them what to do and says she is being rude and disrespectful. I am also 6 months pregnant with our only child together, I am super worried about how it is going to be when the baby comes. His oldest is the apple of his eye and basically rules the house, if she doesn't want to watch a movie, it gets changed, if she doesn't et what is for dinner, she gets something else later, but if my kids want a snack he says they're eating to much.... Someone, please help.
Marriage counseling might be
Marriage counseling might be helpful - but find someone who understands blended families. It's hard to blend 4 kids and 2 different parenting styles. The house rules should be the same, with age-appropriate differences, ie, older kids stay up later and maybe have more screen time, etc. You should each parent your own kids, but back up the other one if it's necessary to parent each other's. The emphasis here, though is on PARENTING. So if no other kid gets to choose the movie, then Princess4 doesn't get to either. He'll have to give up babying his kids and actually give them limits and boundaries.
I think it may be worth it...
To consider whether he has some points. I'd agree that your kids should be held to a different standard because they're older. They get more privileges I assume so they have more responsibilities.
I had the same issue with one of DH's girls (13) bossing around my 9 year old and so that's a particular pet peeve of mine. In my case, I was sitting right there and was aware of what DD was doing. I didn't feel it was an issue so I let it go while SD kept "parenting" DD. I told both her and DH that while I'm there, I'm in charge of DD and if I'm not saying anything, they shouldn't either. That's why I can see your DH's POV with this.
You both have kids that are at challenging ages (except possibly the 14 year old -- my 14 year old son requires little discipline except for nagging him to do his chores). Little kids that age are generally rambunctious and can be rude at home where they can let their hair down. It can be really hard for older kids to deal with their own much younger sibs so I'm sure it's really hard for your kids to put up with "babies" that they probably think are spoiled.
One thing I'd try with your kids is emphasizing how cool it is to get to be role models for the younger kids and empathize with how hard it can be to do that.
Just curious... how long were
Just curious... how long were you with your dh before getting married... he has a 2yr old and you have been married a year.
Did you spend enough time really getting to know one another and see how the other parented prior to moving in? Has something changed?