I agree that not letting her in your bed is imperative, but I'm not sure that saying there's no room because of the new baby will work. It could backfire and make her more jealous and more clingy. My 3 grandchildren had little foam fold-out "chairs" - kind of like a convertible sofa but just their size. It was a TV chair or a chair for their rooms, but it opens up with a quick unfold - a child can do it. You could consider putting one in your room and she has to sleep there.
Another option is putting it in your sons' room - if she's really afraid, she can sleep on their floor without disturbing them. I understand that you are worried about her waking everyone up with her crying and her stubborn streak - but that stubborn streak will be there in some other way if it's not controlled and re-directed. If she won't sleep in your sons' room, then you know it's about YOU and not about pure fear.
You can try letter her sleep with something special of yours - a soft scarf or special blanket - there are ways to print photographs onto fabric and then stitch them on to a blankie or soft piece of fleece. If you don't sew, then use something like Stitch Witchery or other iron-on adhesive available in any fabric store. The fabric store people should be able to give you good advice.
You can consider putting a gate on her door - 2 if she's a climber, one on top of the other - so she can open the door but not get out.
You could try something "drastic" during the February vacation when your boys don't need the sleep as much for school. I know it will be hard on everyone, but it's already hard on you and your husband, and it's not going to get any better when the baby comes. I would definitely not make this about the new baby though - your daughter will probably regress then anyway. She doesn't WANT to be a big girl right now which is why she's doing the clingy and fearful phase.
There are numerous books on sleep habits which others moms will recommend. I think it's important that you and your husband settle on a technique and follow through - whatever you choose won't work in 2 nights or 3, but you have to stick with it rather than abandon it and go to another technique which will just confuse her.
She really needs her sleep so whatever method you choose, be firm and resolute, and don't let her stubbornness deter you. You are tired now and it may seem easier to give in (it IS easier!) but you're going to get more tired as time goes on.
Good luck! It's not easy!