TPGK with obvious draw out, river plan?

[avisco01]

Villain unknown as I haven't been at the table very long. What's the best option on the river here?

Full Tilt No-Limit Hold'em, $0.50 BB (9 handed) Hand History Converter Tool (http://poker-tools.flopturnriver.com/Hand-Converter.php) from FlopTurnRiver.com (http://www.flopturnriver.com) (Format: 2+2 Forums)

MP2 ($43.35)
MP3 ($21.05)
CO ($15.25)
Button ($10)
Hero ($57.70)
BB ($26.15)
UTG ($23.40)
UTG+1 ($38.55)
MP1 ($50)

Preflop: Hero is SB with J/images/graemlins/heart.gif, A/images/graemlins/heart.gif. UTG posts a blind of $0.50.
<font color="#CC3333">UTG+1 raises to $1</font>, <font color="#666666">1 fold</font>, MP3 calls $1, <font color="#666666">1 fold</font>, Hero calls $0.75, UTG calls $0.50.

Flop: ($4) 4/images/graemlins/club.gif, 5/images/graemlins/diamond.gif, A/images/graemlins/diamond.gif <font color="#0000FF"></font>
<font color="#0000FF">Hero bets $2</font>, UTG folds, UTG+1 calls $2, MP3 folds.

Turn: ($8) 6/images/graemlins/diamond.gif <font color="#0000FF"></font>
<font color="#0000FF">Hero bets $4</font>, UTG+1 calls $4.

River: ($16) K/images/graemlins/club.gif <font color="#0000FF"></font>
<font color="#0000FF">Hero ??