Hi Joko,
you need to find your 'Disk Utility' ... it's normally in the Applications folder inside the Utilities folder.
Open up Disk Utility, on the left hand side of the window you should find your Hard Drive. Click on it, and then you should see that you will get a Choice of 4 or 5 Different Tabs (in the middle of the box).
Choose the one called First Aid, then click on Verify Disc. It might take a while to run through. After it's done should be able to chose the Repair Disk option, along with Verify/Repair Permissions. Run through all of those if you can.
Hopefully this will help, but my G4 also recently started to go very slow (just after I'd installed a well known new program) and tho' I did all the repairs and had loads of space left, it never really recovered. How old is your G4 and how much space have you got left on your HD?
Mort