As I've mentioned prior, even down three games to one in the 2009 NLCS, Baseball Prospectus has us at 15% odds to win all three games and take the NLCS title. It can happen, it has happened to 12 teams since the playoffs began, and three teams in the last seven years, if my math is correct.
So say we pulled out that Game 4 after all. I've run the numbers, and my analysis says that only 50% of the teams that are tied 2-2 in a best-of-seven series go on to win the series. You may want to check my math on this, but I've carried the 1 and I think it's true.
It can still happen.