Hello,
Reboot the computer with the CD-ROM that came with your G4. Do not install anything, just reboot with it. Does the screen return to normal brightness and clarity?
If it does, that is great. You have a software config issue. If it remains dim, it could be hardware.
When rebooting, hold down COMMAND-OPTION-P-R , the command to reset what is called the PRAM. If the condition is caused by software, that too should clear it up.
You might also consider going into the display control panel, and changing the Gamma and/or Colorsync settings.
Let us know,
Christian
kc0arf
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