.5/1 My set is crushed here, right?

[Johnes Benjamin]

Villain is running around 10/6 over ~300 hands. I really should have bet the flop but MP3 was a huge donkey so I
guess I was just hoping Villain wouldn't get involved and I was going to c/r donk.
This is my first OP so please help me get it to look right.

PokerStars No-Limit Hold'em, $1.00 BB (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

saw flop|[color=#C00000]saw showdown</font>

MP3 =#A500AF(MP3)/ ($93)
CO ($127.80)
Button ($107.60)
SB =#A500AF(Villain)/ ($99)
BB ($66.30)
UTG ($40.30)
UTG+1 ($173.95)
Hero ($97)
MP2 ($57.50)

Preflop: Hero is MP1 with 4, 4.
2 folds, Hero calls $1, 1 fold, MP3 raises to $4, 2 folds, SB (Villain) calls $3.50, 1 fold Hero calls $3.

Flop: ($13) 4, 5, 6 (3 players)
Villain checks, Hero checks, MP3 bets $6, Villain raises to $18, Hero ???
Call and reevaluate turn is first thing that came to mind. I was pretty sure I was behind but I flopped a friggin' set right??!!! What if I had 55?