If I have a unit at -4 might, and the opponent is casting a spell to deal 2 damage on it, do I have to cast spells to +7 might to keep it alive? Or just +3 is enough (i.e. whether it is treated as 0 or as-is -4)?

+7, not +3. Might modifiers are applied to the unit's actual current value, and −4 is a real value for math — the "treated as 0" rule only kicks in when a spell, ability, or combat summing references the unit's Might (Rule 143.2.b, 143.2.b.1).

So +3 leaves it at an actual −1 Might (treated as 0), and 2 damage marked on a unit whose Might reads 0 is still lethal. You need its current Might above the 2 damage, i.e. at least 3 actual Might, which from −4 takes +7.

The same logic is why +2 on a −2 Might unit lands it at 0, not 2 (FAQ 432 / 8256).