If we call Flight 93 the 4th plane, I've always suspected there was a 5th plane to target the other building but was for an unknown reason not successful.
One attempt was caught by security at IAH trying to board a Continental Airlines flight. There may have even been others, but that’s one I know about (since I worked for that airline). They got caught because they were late for the flight and pitched a fit. So, basically, we can all thank Houston traffic for saving lives.