Great river card, still fold?

[ahnuld]

400 nl on party. I have no obvious image. Villan is normal party 400nl player, meaning not particularly good or tight or lag just geenrally passive. I have A4 offsuit in the BB.
Flop comes A [img]/images/graemlins/heart.gif[/img] 5x 9 [img]/images/graemlins/heart.gif[/img]. 12 in pot. I lead for 10.

! fold, then vilan in CO makes it 25. I put him on a flush draw most likely. I call, turn brings the A [img]/images/graemlins/spade.gif[/img] I lead for 45 into 60 pot. Villan calls. River is the 4 [img]/images/graemlins/heart.gif[/img], my money card becuase I think it made him a flush and my fh at the same time. I check, expecting him to bet his flush so I can CR allin and get his stack. Instead willan goes alln for 320 into 145 pot. wtf?