My friend is declaring an attack on my battlefield, where I have no defenders.

He is attacking for a total of seven damage with three attackers. One of the attackers deals four damage.

I play the card Take Captive, and move his four-damage attacker, who also has four defense, into my battlefield as a defender.

My question is… Since he has already declared his attack for seven damage, will his four-damage attacker STILL deal his damage once I move him to my zone as a defender, essentially doing his four damage on him, killing him?

please refer to your rulebook where the battlefield phase is detailed.

take control of an opposing unit before assigning damage. Then use it as if it were yours.

give it back if still alive at the end of turn.