I would add that the 2 and 4 rules break down the more outs you have, so they are much closer to the true values when you have 2 outs rather than 15. Luckily as you get more outs the accuracy of the prediction probably isn't as important so it compensates for that a bit.

You can also use the rule of 2 on the flop if you just want to know the chances of hitting on the turn rather than both the turn and the river.