Would you cap this river for value?

[caggin]

I play A4s from MP if I have a limper in front.

Why not cap the river? Maybe a tie, but villain may also have caught his flush, not reconizing that he's probably up against a boat. Are you afraid of A8? If he had an A, wouldn't he have raised the turn? Or is he really, really, really passive post flop? In which case I understand your not capping...