The OM-4 has brass top and bottom covers and first generation electronics. The older ones have a battery drain issue, but later on they were improved.
The OM-4T is identical to the OM-4Ti except the T model was for North America. Olympus differentiated the naming to help get gray-market sales under control. Both have second generation electronics and support the high-speed flash sync. They have Titanium top and bottom covers as well as improved environmental sealing.
There is a later generation of OM-4Ti bodies that had improved electronics for CE certification. At this point, the OM-4 naming was eliminated and the OM-4Ti became universal.