Actually, i'd prefer Nolan stay on board after 3, but I dont know if thats likely to happen. Personally, I dont get what's with this whole "trilogy" thing. Everyone always wants movies to be in sets of 3's and it's kinda upsetting. I only suggest Robin for after the third one because anything before that is too early, it has nothing to do with society's obsession over the number 3.
I guess if I had it my way, Dick Grayson would be introduced in the third, with him finally becoming Robin at the very end (not donning the costume unitl the 4th).