Well, the rule of 2 is used post flop. After having made a 5 card hand, the rule is used to gauge the possibility of your hand improving, using the cards that help. The 2% is actually 0.021276( based on fact that after flop there are 47 unseen cards ).
The 4% rule ( 0.043015 ) is based only on if you will see both turn and river, to improve, and is based on combination of 1/47th plus 1/46th. ( after turn, 46 cards remain unseen ).
Remember this 2/4 rule was first created when computers were just coming out, and poker was more "live". Quick math, and tighter players made the calculations easier and more forgiving.
