If Kog'maw attacks a battlefield with a 5 might unit defending it, will the 5 might unit die to Kog'maw's Deathknell after Kog'maw deals combat damage?

No, the 5 might unit will not die.

According to RiftJudge FAQs #7306 and #1082, the game proceeds to the healing step before the Deathknell ability resolves. Because combat damage is cleared during the healing step, the 1 damage from combat is removed before the 4 damage from Deathknell is applied.

Sequence of Events:

1. Combat Damage: Kog'Maw and the 5 might unit deal combat damage to each other. Kog'Maw takes 1 damage (lethal) and deals 1 damage to the 5 might unit.
2. Healing Step: Before the Deathknell trigger resolves, the game proceeds to the healing step. The 1 damage on the 5 might unit is cleared.
3. Death Trigger: Kog'Maw’s Deathknell ability triggers and resolves. It deals 4 damage to the 5 might unit.
4. Total Damage: The 5 might unit has taken 4 damage total, which is not lethal.

Note: While some interpretations suggest Deathknell resolves before healing, FAQs #7306 and #1082 indicate that combat damage heals before Deathknell triggers. I could not fully verify this ruling against the retrieved rules.