The rule of 4 and 2 is the easiest way to know roughly what your odds of winning your hand are.
It's a simple process: first work out how many cards you can hit to improve your hand (outs). For example, if you have a flush draw, there are nine cards you can hit to make a flush.
To know the odds of making your flush on the turn, simply multiply your outs by two. To know the odds of making your flush on the river, multiply your outs by four. In this case, you have a roughly 36% chance of making your flush by the river.