I am not an expert, but as far as I know there never were later OM-4 models that did not suffer from the battery drain problem. There are OM-4 camera's with updated electronics, but Olympus only replaced the boards of camera's that came in for repair. The way to find out if a OM-4 model has an updated circuit board is to switch the camera to battery check mode. If the battery is full, the camera beeps. If the beeping stops after 30 seconds or so, the battery has the updated board. If the beeping does not stop, the camera has not been updated. The OM-4T or OM-4Ti never suffered from the battery drain problem.
Do you know, I think you are right. I used to work for a dealer in 2000/2001 and he would not take in an OM4 2nd hand that had not been modified. I had forgotten exactly how to check.