Does it stay in 4th gear, or does the converter stay locked up? I have never heard of a switch that lets it go into O/D. Just the converter lock up feature.
If it is the converter lock up you are talking about, it is hopefully just the lock up soliniod inside the trans, it is a fairly easy change, or I had a fresh 200R4 get a piece of debris (chip) caught under the plunger that the lock up soliniod actuates, holding it open.
Good luck, and tell us what you find.