Mr Totes is quite right, the 10.1.3 disk will not work. Contact the original seller for the disks that came with the machine. Among other things, they can be used to troubleshoot problems with the machine should they arise (chances are they won't but they can be useful, especially in detecting if the RAM usable and other factors). They are not rightfully his when he sells the machine and ONLY licensed for use with that machine. If he lost the disks, he should call AppleCare for replacements.
Once you have identified which iMac you have from my FAQ:
http://www.macmaps.com/macosxnative.html#IDENTIFY
You’ll have the knowledge necessary to call AppleCare and request the disks yourself if you can’t reach the seller. This knowledge is also important later on.
If you can reach him, the number he should contact is one from his own country.
In Canada, the number for AppleCare is
1-800-263-3394
In the U.S. it is:
1-800-APLCARE
Overseas numbers are here:
http://www.apple.com/contact/phone_contacts.html
Make sure you get the correct RAM according to the spec database based on the Mac model identified above:
http://www.info.apple.com/support/applespec.html
and update it to at least 512 MB if not 1 GB. The installation if necessary is best done by an authorized service technician, especially on the iMac G4 models since those RAM slots tend to be quite fragile. Run the hardware test that came with the restore disks. Some come with their own Hardware Test CD that is silver colored, while others come with a volume like the Powerbook, except labeled iMac restore disk. The restore disk instructions for the Powerbook apply to iMacs as well:
http://docs.info.apple.com/article.html?artnum=86287
If the hardware test appears OK, you’ll be ready to run the Tiger installation with an erase and install:
http://docs.info.apple.com/article.html?artnum=301270
And everything then should work.
Good luck, and let us know if you get stuck along the way!
In the future always be careful when buying used and refurbished Macs, by following these instructions in my FAQ:
http://www.macmaps.com/usedrefurbished.html
