Strange A4s hand... Do you bet the turn or river?

[Guido]

Party Poker 5/10 Hold'em (6 max, 6 handed)

Preflop: Guido is Button with 4/images/graemlins/club.gif, A/images/graemlins/club.gif.
<font color="666666">1 fold</font>, MP calls, CO calls, Guido calls, <font color="666666">1 fold</font>, BB checks,

Flop: (4.40 SB) T/images/graemlins/spade.gif, A/images/graemlins/spade.gif, 9/images/graemlins/spade.gif <font color="blue">(4 players)</font>
BB checks, MP checks, <font color="CC3333">CO bets</font>, <font color="CC3333">Guido raises</font>, BB folds, MP calls, CO calls.

Turn: (5.20 BB) 3/images/graemlins/spade.gif <font color="blue">(3 players)</font>
MP checks, CO checks, Guido checks.

River: (5.20 BB) 4/images/graemlins/diamond.gif <font color="blue">(3 players)</font>
MP checks, CO checks, Guido checks.

Final Pot: 5.20 BB

[Schneids]

I think the turn check is fine.

When it gets checked to me on the river I conclude that MP cold called on the flop with an ace no spade and now doesn't know what to do. For this reason I'd debate a river bet in hopes MP calls and CO folds a baby spade instead of overcalling...

It's not a large pot though so EV wise I think betting for the reason I've said or simply checking are fairly close.