Hi,
if you go to "overdrive", 2 things happen. first, the torque converter lockup clutch is activated and will provide a direct drive thru the converter, eliminating slip. this is noticeble as rpm comes down when the clutch engages. however, locking the torque converter removes vibration damping from the drive train, so that a slight resonance is encountered at appx 1100-1200rpm when accelerating on a light throttle. as soon as the throttle is opened a bit more, the lockup clutch disengages, rpms rise as the torque converter permits "slip".
and the 2nd thing.... only in OD is the 4th gear available. thats why a little reminder lights up in the road speed readout should one forget to change from D to OD and tries to go faster.
D is fine for city driving or winding roads where one might encounter that 1100rpm period more often.
cheers
Wolfram