It took me a minute, but I figured out what's going on. When you're in 1st gear just like SRT SIX said it's kind of a granny gear. When you're in 4, the automatic transmission is taking over and you're probably in 2nd. When you've got your car in D mode most of the time you don't notice the transition between 1 and 2 because it's only in 1st gear for a few seconds.

Just to recap, Just because it say's 4 on the dash doesn't mean it's in 4th gear. When you get to 4 though it won't go into 5th until you either manually put it there or you hit the 6k rpm or close to it.