Is Saboteur better in a 4-player game than a 2-player game?
Saboteur is undoubtedly more dangerous in a 4-player game, but that does not necessarily mean better.

4-player games can end on piles more frequently than 2-player games because you only have 3 Victory cards per person (rather than 4 in 2/3-player) and 2.5 cards/Kingdom pile/person (vs. 5 in 2-player and 3.33 in 3-player). Saboteur can result in up to 3 extra cards leaving the Supply per play, which makes the piles endgame condition much more likely. If everyone plays a Saboteur one turn and everyone gains a $2 card as a replacement, you've almost exhausted 3 piles worth of cards right there—in a single turn.

In the extreme case of all the players going for Sab, you are likely to be hit at least once a turn, and more likely two or even three times. That's the equivalent of facing a King's Court-ed Saboteur in 2-player: you're losing 3 high-value cards a turn, versus only being able to get one your turn. The general 2-player advice against Saboteur is to ignore it because Sab gives no benefit to the person playing it, and so you can build your deck up faster than your opponent can trash it down, and your opponent loses the opportunity to build his or her own deck by playing Sab. The ability to outrace Sab is lost when you're getting hit multiple times a turn, which means that the game is likely to stall at the $3/$4 card level.