It is not like that, Warner Bros has contract with Epic to use UE3 till the end of 2014 that is why Mortal Kombat X and Batman Arkham Knight still use UE3. It is also a smarter move for them to use UE3 to release the game on both current-gen and last-gen platforms (eventhough Arkham Knight is current-gen only). Anyway I hope more AAA devs begin to use UE4 and not just inidies.