When this occurred, how much of Episode 4 had you played through? Was it a straight playthrough from the start of the Episode? Did you notice the save icon appearing regularly on screen? When you exited the game, did you exit through the main menu, pause menu or the Home button? When you restarted your game the next time, did your save say Episode 4? Did it automatically load to this save or was another slot highlighted? If you start your game now, which slot is highlighted?
Any detail you can give can help us resolve this issue, however you may be experiencing the problem outlined in the sticky thread
here. Please see if the workaround fixes your problem.