I'm not a heads-up player so I'm not sure how often you could get away with such gambits. Also, in this situation the opponent might suspect the cheater holds JT, T9, 98, 87, 76, A9, A8 or any number of other hands that could have hit the flop. Wouldn't a solid heads-up player fold A4 frequently enough in that situation that they never realize they're playing against a stacked deck?