Just sat so no reads. Turn I was attempting a c/r, figuring he'd semibluff but he didn't.

How stupid is that river call?

PokerStars No-Limit Hold'em, $0.50 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

SB ($61.40)
Hero ($70.70)
UTG ($39.50)
MP ($27.50)
CO ($53.70)
Button ($78.05)

Preflop: Hero is BB with A, J.
3 folds, Button calls $0.50, SB completes, Hero raises to $2.5, Button calls $2, SB folds.

Flop: ($5.50) 4, A, 3 (2 players)
Hero bets $4, Button calls $4.

Turn: ($13.50) A (2 players)
Hero checks, Button checks.

River: ($13.50) 2 (2 players)
Hero bets $6, Button raises to $12, Hero calls $6.

Final Pot: $37.50