The idea is that you get paid off more when you have a good hand. If someone doesn't raise as much preflop, it's easier to put them on a strong hand when they raise.

That said, I don't think there's anything wrong with 4-6%. If you look at Iceman's stats (which he posted awhile back), he's at 3.7%. His post flop aggression is fairly high, though, so I suspect that's a very important stat if you have a lower PFR number.

Sets with small pairs will be one of your biggest money makers.