If I attack a battlefield with a 4 might unit, and there are two units defending - one unit is reduced to 0 might with a spell, and the other unit is 4 might. Can I kill both units?

No, you cannot kill both units in this scenario.

Reasoning

Under Rule 460.2.c, combat damage is resolved by summing the current Might of your units at the battlefield and assigning that total as damage to the opposing units. In your scenario, your unit has 4 Might, and there are two defending units:

1. Unit A (0 Might)
2. Unit B (4 Might)

During the Combat Damage Step, you must assign your total damage (4) to the defending units. While you can choose to assign damage to either unit first, you are governed by the rules for lethal damage assignment:

• Lethal Damage Definition: Per the rules, lethal damage is any non-zero amount of damage equal to or greater than the unit's current Might.
• Unit A (0 Might): Because its current Might is 0, any amount of damage assigned to it is technically "lethal" (if we consider the game's requirement for a non-zero amount). However, if you assign a positive integer (the minimum allowed, which is 1) to Unit A, it receives lethal damage and is destroyed.
• Assigning Remaining Damage: After assigning the 1 damage required to kill Unit A, you have 3 damage remaining. You must then assign that remaining damage to Unit B.
• Result: Unit B has 4 Might. Since you only have 3 damage remaining to assign to it, you cannot reach the lethal damage threshold of 4. Unit B survives the combat.

You cannot split the damage in a way that kills both because you lack the total Might necessary to satisfy the lethal damage requirements for both units simultaneously. You are required to assign lethal damage to the first unit before assigning damage to any other unit, and once Unit A is accounted for, the remaining damage is insufficient to destroy Unit B.