PokerStars No-Limit Hold'em, $0.25 BB (6 handed) Poker-Stars Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver Cards)

Button ($26.45)
Hero ($40.40)
BB ($31.75)
UTG ($96.35)
MP ($23.05)
CO ($13.25)

Preflop: Hero is SB with , .
4 folds, Hero raises to $1, BB calls $0.75.

Flop: ($2) , , (2 players)
Hero bets $1.5, BB raises to $4, Hero ????


What is the correct move here?