You shouldn't need a fresh install, they dont suffer the same rot as a windows PC.
Currently retail copies of Tiger on ebay go for close to 100 dollars, which is a bit pricey for an old version.
Did the previous owner give you their admin password. If you have an admin login, you can create a new user in the Accounts preferences for yourself.
That new user account will be clean and fresh. You can also simply trash any apps from the main applications folder, install your own stuff and when everything is set up for your needs, you can delete the old admin account (make sure your new account is an admin first)
So, if you have admin access there is no need to re-install, just create a new admin user and make the G4 your own. If you want to give OSX a refresh after that download the combo updater and run it.
Apple - Downloads - Mac OS X Updates - Mac OS X 10.4.11 Combo Update (PPC)