how do i mount this old Toyo Toki (Toto) sink?
The two top holes in the corners are there because the sink is hollow. They are from the making of the sink. You will not use those holes. Every wall mounted sink that I have installed uses a bracket that is mounted on the wall first into at least one stud, then the sink drops down onto the bracket. The two lower holes then would probably line up to two holes on the bracket so the sink cannot be accidentally moved or bumped off the bracket. 

If you do not have the bracket, you may have to fabricate one (I would use wood). It should not get wet back there if you caulk along the top of the sink after installation, so the wood should not fall apart over time.