Sure, 15 x 4 = 60%, which is a bit off from the actual 54% you're actually getting, but that's a pretty minor detail as it's pretty much an instant call getting better than 50% odds to win a hand.
Here is what you seem to fail to understand: the rule of 2 and 4 is a way to get a quick estimate of the percentage chance your hand has to win. That's it, it's a quick guidenothing exactly 100% but a fast mathematical method to get close enough.
If you want, in some parts, you can +n/n after some of the x2/x4 to get a bit closer to the actual percentage.
The way it is applied is meant to be a quick and easy way to see if you're getting the proper odds to call a bet. Lets say, for example, I have 9 outs for flush:
9 x 2 = 18 or a ~18% chance to win. Actual odds is 19.1%
Opponent bets $30 into a $90 pot. I have to call $30 to win $120. $30 is 25% of $120, so the pot odds is 25%
I know 25% pot odds > 18% hand odds, so it's an easy fold.
I did not need to know the exact 19.1% hand odds in order to make this decision.
The rule of 2 and 4 is a guide and nothing more. For more accurate hand odds v pot odds, do the ratio method, but do them correctly. Both method works, but be sure you're doing them correctly when it comes to comparing hand/pot odds and when you need to call/fold. For example, don't do percentage method for hands odds, ratio for pot odds unless you can convert one of them into the other. Ratio: Pot odds > Hand odds = Call. Percentage = Hand Percentage > Pot Percentage = Call
