Can you play Gust targeting a 4 Might unit and then react to it with Stupefy to reduce the unit's Might to 3 or less before Gust resolves?

Ruling: No, you cannot target a 4 Might unit with Gust. The targeting requirement "a unit at a battlefield with 3 might or less" must be met when the card is played, before any reactions can be added to the chain.

Sequence:

• Stupefy must be played first to reduce the unit's Might to 3 or less
• Then Gust can be played targeting the now-valid unit

Nuances:

• If Gust were worded "return a unit at a battlefield to its owner's hand if its Might is 3 or less," then you could target any unit and the Might would be checked on resolution
• The "3 might or less" restriction in Gust is part of the targeting requirement, not just an instruction that resolves later