2 things most people have failed to acknowledge.

1. An ace may only have 4 outs, but he has 2 chances to make those outs. The turn and the river. He is ~5:1 to win this pot on the flop.

2. His chances don't diminish on the turn. Any turn card gives the Ace another 3 outs with runner-runner boat. He is now 5.6:1 to win the pot.

EDIT: It doesn't matter what the turn card is (a 2,3,4 pairing still gives villian a bigger boat).