No No, maybe I say it more clear:
You start with 4, and 2 leave. So you are two left. To get your score counting, you both guys, that are still ingame, must die! So that the end music plays where you see the stats. Then it will count! So even when 1 or 2 guys left, you still can do a highscore attempt.
The only thing will be, that it will say "4 players" in the leaderboard, just cause you started off with 4 players. And the leaderboard is made so that when there is a 4ppl and a 2ppl game of the same round, the 4ppl will count weaker.
Yeah as long as the two 2 people left aren't you and your split-screen partner.
My wife and I broke down and bought another copy of black ops so that we could play together (and as long as one of us is host) without worrying about people dropping out and screwing us up. We already had two ps3s so it made perfect sense
Hopefully with the next game we get some more features on this front and better leaderboards.