E.C.
I would have the 4yr old sleep with the 12yr old and put baby in his crib. I would think it would be hard for both of you in the playpen in your room because you will be right there. If he sleeps pretty much during the night with you he shouldnt need to eat every 3-4hr if he wakes during the transiton also. It is going to be a hard few nights but if you stick with what you have decided to do for a routine he will get it and it will be easier. Good luck.