We've come to accept that The Dark Knight Rises will likely be the last time we'll be seeing Christian Bale as Batman and Christopher Nolan directing the franchise. Yet there is glimmer of hope from Christian Bale via Shockya who mentions the possibility of Batman 4 happening in the future but only if Nolan is on board.
“Until (Chris Nolan) tells me (there will be a ‘Batman 4′), I don’t believe it. It’s gotta be from his mouth.”
I'm sure Warner Bros. will try their best to sign Nolan and Bale for a fourth film depending on how well DKR does. Bale had been obligated for the third film but it wasn't until few months back that Nolan had finally signed on. I wouldn't be surprised to see Warner Bros. offer the director the option to make any new Batman films when he feels like it. Allowing him to focus on his own original films which are very successful in their own right.