If your OD is wired correctly and the 4th gear switch is functioning by not closing in any gear but 4th, when you shift out of 4th gear, the OD looses power and drops out of OD mode.

To check this function, with ignition ON, engine OFF, in 4th gear, "engage" OD.

You should hear a click from under the auto as the OD solenoid actuates. "Disengage" the OD and another click should be heard.

Have the OD on and move the shift lever out of 4th. If the switch is working, the solenoid should click "off".

With your vehicle, if the OD is left ON as you downshift, in a factory wired and properly working system, the worst that would occur is when you got back up to speed and shifted into 4th, the OD would engage and you would have gone from 3rd to 5th. Probably feel a little sluggish at that point. Depending upon engine RPM's when the shift occured, it might lug the engine a bit until RPM's get back into the engines power range.

Duane