The OM4 is a reliable camera, but it is always in respect to how the camera was stored and used.
I have an old popular photography article where they btested the OM4 with heat, cold, sand, water, and a drop and it came out great in almost every test. I think one of the extreme temperature tests it stopped working for a minute until it returned to a lest extreme temp. It is very well sealed against dust and dirt, but not water proof of course.
That being said..
Smaller cameras are miniturized in every way.. that means gears, levers, electronics are smaller than their larger counterparts.
Small gears and levers will wear and warp more if abused. Electronics get shoved into small places and are delicate.
And any camera that has not been serviced for 20 years will probably need a CLA to be in tip-top shape.
The battery issue is the main problem ive seen. The OM4 i have has the later circut board, so it doesn't have that issue.
Another problem is the LCD readouts can dim or go bad over time. Dont know how often that happens on the OM4. Just a general thing.