Hi
I'm not sure whether you've ever swapped the graphics card in your computer, but it could be the problem.
The MDD G4s originally shipped with an nVidia GeForce4 MX or Radeon 9000 Pro, with 32MB and 64MB of VRAM respectively, both capable of supporting resolutions of at least 1600 by 1200.
The Rage Pro in your computer was shipped in much earlier G4 models and I'm not sure it supports the resolution of the 204B.