bedeviling spirit is revealed from hidden when there are 4 units on the battlefield. the opponent gusts it. does its ability finalize fully (bouncing the enemy unit)?

No — the trigger still resolves, but it whiffs, so no enemy unit gets bounced.

The "When you play me" ability still goes on the chain and resolves even though Gust returns Bedeviling Stranger to hand first (FAQ #3430). But a play effect of a permanent that was played from hidden references where its source was played to in order to determine its legal targets. Once the Stranger is no longer on the board, the effect can't reference the battlefield it was played from, so it mistargets and does nothing (Rule 811.1.d.3.a).

So the ability does finalize — its battlefield/unit choices lock in at finalization, while the Stranger is still present — but by the time it resolves, the source is gone and the return-to-hand instruction is ignored entirely.