if i play a card that reduces 4 might and target a 2 might unit, then what might is that unit

Yes — it ends up at 1 might, not negative, and it doesn't die from the reduction itself.

Every might-reduction card currently in the game (Smoke Screen is the -4 one) reads "to a minimum of 1 might," so the effect is limited at the moment it applies: your 2-might unit only takes -1, and that -1 is "snapshotted" for the rest of the turn (rule 479.4.b). If something later gives it +2, it'll be at 3 might, because the reduction stays locked at -1.

One caveat: if that unit already has damage marked on it equal to or above its new 1 might, it now has lethal damage and will die in the next cleanup — but the might loss alone never kills a unit (FAQ 5467).