Set on a flush board--what to do?

[Chicago Kid]

MTT-Middle Stages

Blinds are 400-800

Hero (32K), one off the button, is dealt 4 [img]/images/graemlins/diamond.gif[/img] 4 [img]/images/graemlins/club.gif[/img]

UTG (45K) limps in, MP (26K) limps in.

Folded to me. I also limp, in "No set, no bet" mode.

SB mucks, BB (smallish, but can't recall) checks.

I've played with UTG on several occasions, and know him to be an aggressive player pre-flop, so his limp seemed suspicious.

MP is new to the table and BB is somewhat rockish. Not many hands played, generally has the goods when called.

Flop (3600) comes 2 [img]/images/graemlins/spade.gif[/img]4 [img]/images/graemlins/spade.gif[/img]9 [img]/images/graemlins/spade.gif[/img]

UTG bets 5K, MP mucks.

What's your play?