You would win the game in this situation. State based effects would see that he had 0 or less life and the game would be over with the reaver's triggered ability on the stack.
Yes damage dealt is not dependent on anything's toughness. The only time that this is true in drain life type situations, when you can't take more life than the creature or player has. So in this case you would still take 4 when the reaver's trigger resolved.
RJ's answer is correct, but I'm going to nitpick on a few technicalities (new rules change on one, and rarely but sometimes relevant ordering on the other).
1) "State-based effects" no longer exist. They're now State-Based Actions. They still work the same way, they just have a new name.
2) State-based Actions are checked before the triggered ability goes on the stack. This is what allows two Sharuum, the Hegemons to recur each other infinitely; they're put into the GY due to SBA before the triggered ability goes on the stack (and it's target is chosen).
Also, "Drain life type situations" can be a bit misleading. Drain Life, and a couple other cards, specifically state a limit on the life gain. It is not something true of all "Drain life type" cards. Consume Spirit, for example, does not have such text.
