Lemme help here, Cover:

OP, if it's your DH's time, it's his responsibility and right to spend it with his daughters. If one of them acts up (and she's 4, weddings are going to be boring about the flashy wears off), then he needs to act as a parent and take care of them. Your want for him at the wedding with you does not trump his need to parent if his kids misbehave.

Now, what you CAN be upset by is if he does nothing in the next 3 months to curtail SD4's behavior BUT insists that she goes. He can't just not parent her but reward her with stuff she wants. That's not an SD4 problem, though; that's a DH problem.

I say make it clear to him that SD4 only gets to go if she can behave appropriately (note that I didn't say perfectly; she's going to likely throw a tantrum at some point in travel and festivities because she's 4), otherwise he needs to figure out another solution BUT you expect him to be at the wedding with you and SD6.

Now, how he accomplishes this ask is up to him. He can actively parent SD4 in the next 3 months in such a way that it's effective. He can beg BM to keep her while he takes SD6 to do something fun (which seems cruel). He can hire a babysitter to was SD4 at the hotel.

Whatever choice he makes needs to limit your stress because it's your family and SD4's stress because he's her father. This means having BM travel with you all like a sister-wife isn't going to fly because that increases your stress. This means he doesn't get to just take SD4 and say "it'll be fine" because that increases everyone's stress. He has to ACTIVELY do something.

If he can't accomplish any of this, or just says he won't go, then you have to be prepared to start doing things on your own. There is no other solution other than no longer being married to him.

Now, I say ALL of this assuming SD4 is not just your typical 4 year old, whose just emerging out of toddlerhood, and therefore still gets overstimulated and can break down. I'm assuming that her misbehavior is constant and that your DH is ineffectively parenting. If, however, she is acting like a typical 4 year old and you just don't like that, then you need to skew your expectations to what is appropriate. I'm assuming, though, since DH is allowing a 4YO to dictate her visitation that your DH is just a poor parent.

Basically, your DH's ability to ineffectively parent his daughter will impact you. So, what are you willing to give up for that? Is your DH willing to parent better? Are you okay with staying with a man who abdicates his responsibility to BM because parenting is tough and he's tired? Are you okay with traveling on your own, or traveling with a 4YO who isn't going to behave well? Either your DH gets better at parenting so that SD4 behaves in a way that doesn't make you resent her, or your DH stops visitation with her so that you don't have to deal with her, or your DH does nothing and you start being okay with doing things alone. Those are the only real options, unfortunately.

But they ARE getting a say. It isn't SD6's responsibility to talk her sister into anything. It is your DuH's responsibility to take his time with his daughter and parent her? That means that he, as the adult and parent, does not let her decide when she'll come over and what she'll do. She's FOUR, for goodness sake!