I think if it was me I would work on getting the 11 month old and the 4 year old both into one room - is that an option for them to share? Spend some time - especially with your DD - getting the room really cozy and welcoming and comfortable. I would get her some ear defenders, and practice putting them on when the baby is crying. Set up a whole new bedtime routine. They could have a pretty nightlight so that it's not dark, some soothing lullabies playing on a loop. Equip your oldest daughter with a new special toy and give her things she knows she can do if she wakes up int he night. Maybe set yourself up with a mattress on the floor in there too - you can lie down and rest there in the beginning stages, and later DD can sleep there so she feels close to you. Explain why this needs to happen - because mummy needs rest before the new baby comes.
With the 11 month old, Erin has already given you great advice. I would keep the dummy because he can control it himself and it will be comforting to him to have it when he's transitioning to this new way of sleeping. Yes there will be a lot of crying so you will need to feel strong but you can totally do this yourself. Let your DD know that DS will be crying because he doesn't know how to go to sleep, but she can wear her ear defenders, and mummy is nearby, and her job is to stay quietly in her bed. She could have a torch and some books to look at. I understand this is all easier said than done but what I'm trying to get across is setting them both up for success as much as possible.
My two have a four year age gap and they LOVE sharing a room together. My son loves that he is there 'taking care of' his little sister and she gets comfort from knowing he is nearby. Now that's she's 2.5, when she wakes up she will often call to wake HIM, and then ask him to come and get mummy or daddy (she is still in a cot so can't come herself).
I hope that helps to see there may be a way through this. Please ask questions and come back for more support as you work this out.