When in D mode, does the screen have a number that indicates the selected gear? I think that's a feature in the MY09 and after cars, so if your car was built before April/May 2008 it may not have it. Also, even in D mode, if you press the gas pedal with a bit more power it upshifts, but in a sleek way, sometimes you don't even know. That basically means that it could have gone into 5th and you didn't notice.