I don't know how exactly gp4 makes the difference between lowres and hires. It can be either :
1. The resolution you're playing on 800*600 (low) -> 1280*1024 (high)
2. The number of detail you 've turned on
3. Depending on the PO values, GP4 might be autoselecting low or highres
4. A combination of the above options.
This is only a wild guess. The only thing I can say about this is that you can figure this out by trying different settings.