NL25 99 preflop

[HighCardSeven]

Poker Stars - No Limit Hold'em Cash Game - $0.10/$0.25 Blinds - 6 Players - (LegoPoker Hand History Converter)

BB: $27.00
Hero (UTG): $25.00

Preflop: Hero is dealt 9 9 (6 Players)
Hero raises to $1.00, 4 folds, BB raises to $4.00, Hero ???

Villain was unknown. The raise from him is very big. I can not call that for setvalue, right? So I should fold? Or should I call and try to bluff him out sometimes on the flop. Maybe he is raising with AK or AQ and if he do not hit the flop I can use my position to win the pot.
What should I do against an unkwon player?