If I'm understanding correctly you want to know why the auto trans goes from 4 to D and not 4 to 5. That's because it is only a 5 speed transmission, when you shift up from 4th gear you're in D which is also 5th gear. If you want to leave it in D then it will do all the shifting for you from there or you can continue to pretend you're driving a manual and downshift into 4th.