If the chosen target is an illegal target by the time Sorin's second ability tries to resolve, the ability doesn't resolve. You won't gain 4 life.

The control-change effect from ability granted by Sorin's last ability lasts indefinitely. It doesn't wear off during the cleanup step, and it doesn't expire if Sorin or a Vampire whose ability was activated leaves the battlefield. In a multiplayer game, it does expire if you leave the game.