Sammy, I think what Jesus is trying to say is that, as the Rule of 4 is usefull in determining your pot odds with 2 cards to come, it is in effect useless in all practicality because you should only be concerned with your odds of hitting the next card. Well, I wouldn't go so far as to call it useless, but technically he's correct. Its variation, the Rule of 2, is equally convenient for that purpose. But lets not dismiss the big brother. There are several common situations where you need to figure your pot odds with 2 cards to come. Here are some examples.
1) The most obvious application is the forced All-In. Are the pot odds favorable? Remember, once you call, your opponent can't make you bet again. 2) This is my favorite! When the 1st bet will leave you short stacked, it is often the case that when you miss your draw, your opponent will no longer be able to raise enough to make a final call unproffitable. You may in fact get far better odds than you need to see the river. This should be taken into account on your previous bet. Unless your playing with Chris Ferguson it's unlikely that your opponent considered this. 3) Your Implied Pot Odds, a more abstract concept which XDMANX touched upon, justify the 2 combined bets. Implied odds are tough to figure. There's no cute math trick for this. It's really more of a confidence level that, when you draw out, the future size of the pot will justify your first 2 bets. In short, when you hit, your gonna rake it in! How the heck are you supposed to know that? Good 'ol poker instincts, my greatest failing.