Oh, right.
All right, if we wind up with 4 cards, 1 action, plus whatever Ironworks gives us, I suppose we have a wider selection of choices. In that case, if Ironworks gets us an action card, we're up +2 Actions only, which is a Native Village (taking zero cards off the mat).
Ironworks can't take an Estate except in a 5+ player game (because we need all the 4's to be out), but in that case we'd have 5 cards, 1 action, in which case the answer could be a Great Hall, an inactive Menagerie, a Caravan, and probably other things as well.
I think the only other Victory card that works is Great Hall, which puts us at 5 cards, 2 actions. That's a Village -- Walled, Worker's, Mining, or vanilla.
For Treasure cards, Ironworks could snag us a Silver or a Loan; I don't think any others work. In that case, we're at 4 cards, 1 action, and 1 coin. Maybe I'm blanking, but I don't think any single card gives us that. Herbalist is close, but it leaves a +Buy unaccounted for.
Anyway, those are my new answers: any of several plain cantrips and Villages.