I thought I was in the movie section, and I was trying to remember whether The Sign of Four was ever a movie. Duh, Sign Up, Four (women)...
Oh!
Designing Women
Chain of Command, the NG ep where Picard is taken captive by Cardassians. one of whom tortures him to try to get him to say he sees five lights when there are only four. Picard has been rescued at that point and is on his way out the door when he yells this at his captor.
Species 8472 was the sworn enemy of the Borg, so it could have been Enterprise (never watched it), but could definitely have been Voyager, on which the species was introduced.
No clue what episode.
Can't use Wall Street. We just went over that rule: you can't go back to the same movie.
Martin Sheen has a cameo in Hots Shots 2, which starred Charlie Sheen. So to fix sudossuda's link:
Hot Shots 2
Charlie Sheen
The Three Musketeers