A solid heads-up player will pretty much never fold A4 from the small blind. small blind is also the button, so they'll have position for the rest of the hand.

Really, the only question is how much to raise, or if to limp. the limp/mini-raise tactics would be there essentially to add some deception to post-flop play.

Ace high is a pretty strong hand heads-up, even with the shit kicker, as the odds are against your opponent flopping any pairs at all.