Is this NL or Limit? If Limit then forget it (I say that cause one more bet usually can't bluff someone out), NL you can actually bluff if the flush hits the board on the turn or river. The raiser might very well have hit top pair on the flop and with a pair of 4's (your head up like you said after he bet the flop) you could raise right then to find out it he does have top pair or not but I know this isn't about that. Actually this play will present itself when you have either top pair or second best or even a straight draw that never hits where your opponent is taking the lead and your just calling him down. Then a flush draw hits on the river and then you raise him good. Just as long as the flush draw started from the flop will it work. So you still have somekind of hand unless your were really on a straight draw.