Well, I might suggest switching the sleeping arrangements, perhaps putting the 4 month old in his/her crib in your bedroom until you can help the 2 and 1/2 year old get used to sleeping on his own, without awakening the baby.
Could your husband accompany the two year old to his bed and lay with him in his own bed to help save your back? That way your son wouldn't be leaving his room, but still get the comforting he needs to sleep on his own?
Maybe you could also introduce a lovie or stuffed animal that might be extra security for him at night. one thing you could do is help him to feel more independent for instance, during the daytime you could talk about waking up from his bed and coming to Mom and Dad's room and decide how he could deal with this in the future. What I mean is something like - "sometimes when Mommy wakes at night I need a drink or to go potty, then I get back into my bed and fall asleep what do you need to do when you wake up at night? How could you go back to sleep in your bed? could you put your (insert fave toy or lovie here) to bed and then help him sleep in your bed?"
you may have to separate the infant to your room and spend a couple of nights working with the 2 year old and getting him back in bed on his own with some fussing. I'm not a cry - it- out advocate, by any means, but you may need to have to help him gently and firmly reestablish his sleeping in his own bed with a few tears.
Good luck!