Felt like completing this for some reason. Action on river?

Party Poker No-Limit Hold'em, $ BB (4 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

saw flop|saw showdown

UTG ($48.55)
Button ($48.31)
Hero ($52)
BB ($50.50)

Preflop: Hero is SB with 4, 5. Hero posts a blind of $0.25.
1 fold, Button calls $0.50, Hero (poster) completes, BB checks.

Flop: ($1.50) 4, 2, 5 (3 players)
Hero bets $1, BB folds, Button calls $1.

Turn: ($3.50) 4 (2 players)
Hero checks, Button bets $4, Hero calls $4.

River: ($11.50) A (2 players)
Hero ???