If your G4 if a Yikes, Sawtooth or Gigabit ethernet then the plastics will fit 100% but if you have a digital audio or newer then the back is different as it adds another PCI slot. The Quicksilver and MDD are very different front and back.

The side panels from all will work on all and the top panel should also. The fronts and backs are what could possibly not fit for you.
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Mac mini C2D 1.83GHz - 2GB RAM - 200GB 7200rpm HD
Sawtooth w/G4 1GHz - 1GB RAM - SIIG SATA w/2x1TB HD
B&W w/G4 600MHz - 1GB RAM - 2x80GB HD
Sawtooth G4 400MHz - 512MB RAM - 40GB HD
Dell Ultrasharp 2007WFP 20" Widescreen LCD
2.94TB Total Storage