A small space idea, like a tree house in your room. Utilize all space with shelf stairs up to the bed box. Get a closet too and all of it up off the floor space. Great for a boys room. Put a BB Net on the bed and voila! they can play indoors.
It is ideal to have enough space so each of your child has its own bedroom. But what if you have just one bedroom and four kids? The best way out is opting for bunk beds that will accomodate all of your kids plus will help solving space issues.