Rooms?

I currently have a 4 year old girl and a 2 year old boy. We just found out we're expecting boy/girl twins! How should we set up rooms?!
At the moment, my son and daughter have their own room. And we also have our guest room available. Would you put the boys/girls together? Or the twins together?
I can't decide because I'm a HUGE decorator! And I can't decide on a boy/girl nursery. :/