where is it?

 

There is no Overdrive like in other cars, where they have a button to engage and disengage. The "G" has 5 gears, the only time that you "drop" down into 4th is when moving from Automatic to Manumatic, it is programmed to always go to 4th, to prevent going to too low a gear and damaging the tranny.
If you want to only shift into 4th and hold it just leave the Manumatic in #4 and that is the highest gear it will shift up to, also any # you leave it in is the highest gear it will shift to, so be careful and remember to shift up. Takes some getting used to w/o a clutch since the "feel" isn't there and the "G" revs to redline pretty fast.

Merlin.

 

the ecu detects when it needs to downshift on its own.