The moment you have gained enough XP, FO4 levels you up immediately. That can be distracting and annoying when you happen to be in the thick of battle. FO3, however, doesn't do that. It waits until you finish the battle and are idle before notifying you with the level-up screen and music. A small detail, to be sure, but I wonder if FO4 can be patched or modded to behave like FO3 in this regard.