Party Poker No-Limit Hold'em, $ Hero (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

MP2 ($184.51)
MP3 ($70.85)
CO ($117.75)
Button ($45.61)
SB ($102.20)
Hero ($97)
UTG ($60.17)
UTG+1 ($41.45)
MP1 ($59)

Preflop: Hero is BB with A, K. SB posts a blind of $0.50.
2 folds, MP1 calls $1, 1 fold, MP3 raises to $4, CO calls $4, Button calls $4, SB (poster) calls $3.50, Hero raises to $18, MP1 folds, MP3 folds, CO folds, Button calls $15, SB folds.

Flop: ($51) 9, 4, Q (2 players)
Hero ???

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Villian is an unknown, have only observed him for an orbit though, but he has been playing many hands.

What's your move here?