If a 4 might unit is hit by Bellows Breath and then targeted by a spell that resumes might by 3 or more, does the unit die?

Yes, the unit will die.

In Riftbound, a unit is killed if it has marked damage equal to or greater than its current Might during the combat cleanup or whenever damage is checked after an effect resolves (such as a Might reduction).

Here is the breakdown of why the unit dies:

1. Marking Damage: When you play Bellows Breath, it deals 1 damage to your unit. This damage is "marked" on the unit and remains there (it does not reduce the unit's Might).
2. Might Reduction: When the second spell resolves and reduces the unit's Might by 3 or more, the unit's Might is lowered.
   • If the unit had 4 Might, reducing it by 3 brings it down to 1 Might.
3. Lethal Check: After the spell resolves, the game checks the unit's status. Because your unit now has 1 marked damage and its current Might has been reduced to 1, the marked damage equals its current Might.
4. Result: Per Rule 428.1.a.2, the unit is killed because it has lethal damage marked on it.

This interaction is consistent with RiftJudge FAQ #9019, which confirms that if a unit is damaged (marking damage on it) and its Might is subsequently reduced, the unit will die if that marked damage now meets or exceeds its new, lower Might value.