OP Not kept up with all the comments but this stands out for me.
"Honestly, I don't particularly see it because I'd be happy for my boys to share with anyone, but they are boys. Perhaps that's why I feel differently?"
Well...
That is how you feel, may not be how the boys feel
The older the child the less likely they are going to be keen to change clothes in front of a member of the same sex, whether they are related to them or not
Generally girls are more self conscious about changing in front of boys than vice versa, perhaps because they develop earlier, perhaps because they have more to cover up or perhaps because they feel conscious that their bodies will be viewed differently. (Are you really not aware of these things?)
The boys are in their own home. If they feel unhappy or frustrated they know where to go to get some piece or quiet. Whether that is the spot in the garden where there is a bench, the corner of the utility room where they can sit on the floor and play on a device or read a book undisturbed, or the fact no one uses the dinning room after 8.00 pm, or whatever. The girls will only be there 4 days a month and so it will take a while for them to know these things and even longer for them to feel at home there - if ever.
When I adopted our son one of the requirements was he should have his own room. It's a no-brainier as he is a boy and much younger than our birth dd. But if they had been the same sex or closer in age it would still have been a requirement that he have his own room. So he can go and find a bit of peace and quiet if he wishes, etc.
You do not seem aware that the girls may not feel 'at home' in your home or that an idea (presumably you have not had them all over for a night yet) may seem great to kids but feel different when they are actually living it.
I'm not having a go, I just feel you don't seem able to see beyond your own or your boys' potential feelings and see how an 8 year old girl may feel being put in a room with her two sisters and two unrelated boys.
Wormulonian "I don't see why the the girls can't sleep in the living room " Because then the girls would really have no bolt hole if life in a blended family of 9 for four days a month all got too much!
"We've been together 8 months. They've been split over a year." That is very fast. I am surprised you think this is a good idea to move in and then have the kids around this. I know you have known him a long time but being in a relationship is different to friendship, as you know, so it does seem very fast indeed.