But if the stepkids are only there one night a fortnight, why is it an issue if the teenage girl shares a room space with the baby as long as that two nights a month she's physically present you have your baby actually sleeping in with you? Presumably it's a non-working night for you and your partner so it'll be a treat for the baby, won't affect the dsd apart from having her baby bro's things in her room, and by the time your son is old enough for her to really find him intrusive, she'll have left home anyway. And if it's just one night a fortnight then I do think the onus is on the adults to make sacrifices on that basis, not the stepkids.
You can get a high sleeper bed for her, some have shelving up there so the baby won't be able to access her books etc and space for changes of clothes. You could even put an iPod dock there or a little light - the leads can be safely hook-tacked in the inside length of wood of the high sleeper, against the wall so the baby never gets to them. Get her some pretty bedlinen she chooses from IKEA, and make that wall a feature wall she's helping you choose the paper for - basically stress it will be her space too, but you don't have the money for 4 beds so want to ensure she and her brothers get space in the house as well as the baby. Involve her and offer that as a way she has her own room when she's there, but below her high sleeper corner is your ds' play area as he is smaller and needs floorspace in a way big kids don't. Obviously as your ds gets bigger he needs to have it emphatically explained that it's his sister's space up there and he must respect it, but that's no different to any other siblings. And most of the month he'll still benefit from a nice big bedroom primarily to himself.
It makes sense to give the boys one room with bunk beds as it'll be the smallest, and then split the bigger between the dsd and your son, to me. If you can't afford 4 beds you can't, but it's unfair to ask 3 mixed sex teenagers to share in their own dad's home, I think.
Ask her, talk to her, explain the issue and that you know she's too old to share with teenage boys, so would she mind if her things are in with the baby, as long as she doesn't physically have to sleep with him in the room unless she wants to. And some kids are really fond of little 'uns - she may like sleeping with him there too while he's weeny. Talk to them, see what they say, explain the issue but please don't suggest they bunk in together while the little prince has his own room. If they offer that, lovely. Please don't ask. Look for workarounds.