aytopper, the problem with the 4x rule is that it is only really relevant when you or your opponent is all-in on the flop, because then there are guaranteed to be no further bets on the turn (or river).
If both players still have decent stacks, you can't follow the 4x rule because the villain may well make another bet on the turn. So you need to think of good odds as following the 2x rule on both the flop and the turn.
(then, of course, implied odds become relevant - but that's another topic)
