If I play Gust on a unit and my opponent uses Reaction Speed to increase the might of that unit to 4, does that unit still get gusted?

No, the unit will not be returned to its owner's hand.

According to the rules of target legality in Riftbound, a spell must meet its targeting requirements both when it is played and when it resolves.

The Sequence of Events:

1. Initial State: You play Gust targeting an enemy unit with 3 Might. Because the unit has 3 Might or less, it is a legal target. The chain is now in a "Closed State."
2. Opponent's Response: Your opponent plays a card or ability to increase that unit's Might to 4.
3. The Chain (Bottom to Top):
   • Gust (Bottom)
   • Might-buffing effect (Top)

Resolution (LIFO - Last-In, First-Out):

As per Rule 359.3.e.7, the top item on the chain resolves first:

1. Buff Resolves: The target unit's Might increases from 3 to 4.
2. Gust Resolves: Before the effect ("Return a unit... to its owner's hand") is applied, Gust re-checks the target's legality. Since the unit now has 4 Might, it no longer meets the targeting requirement of "3 Might or less."

The Outcome: Because the target is no longer legal at the time of resolution, the instruction to return it to the hand is ignored, and Gust has no effect on the board state. This is supported by RiftJudge FAQ #36 and FAQ #10710.