ok, so this is a full ring table that is playing short, both villains bad.
villain in this hand is 50-10 over twenty hands, playing one table, non-reg, likely uber-fish. He likes to bet sometimes, and probably wants to get even with me. I've been an arrogant mofo and suspect a post-flop edge. I've also shown down weak pairs a couple of times - but in very small pots.
Raise pre - sometimes, I like seeing cheap flops against these type of players though.

Note villains stacksize. Monkey checked to, monkey bet, right? and this is never, ever check-call?
Full Tilt No-Limit Hold'em, $0.50 BB (3 handed) - Full-Tilt Converter Tool from FlopTurnRiver.com

Hero (SB) ($91.45)
BB ($20.05)
Button ($20.90)

Preflop: Hero is SB with A, 4
Button calls $0.50, Hero calls $0.25, BB checks

Flop: ($1.50) 4, 2, 9 (3 players)
Hero bets $1.50, BB calls $1.50, Button calls $1.50

Turn: ($6) 4 (3 players)
Hero bets $3.50, BB calls $3.50, 1 fold

River: ($13) 6 (2 players)
Hero checks, BB bets $5, Hero raises to $20.50, (enough to put villain all-in)