If you have ram stick installed, try removing it. Instructions are easy to find if you need them, Google, YouTube etc.
If you have OS X disk, try booting from it and using Disk Utility to repair disk.
Just to make sure, reset Pram and Pmu, smu or whatever it was called on that era. Instructions from Apple website.
Do you know what model iBook G4 you have, year or processor/graphics card model? Different models had different issues, like GPU going bad or Airport connector going bad, from these two it could be the latter. Try booting in safe mode, power up and press Shift -key and keep pressed.