If you own an old G4 iBook it is probably running on a version of Mac OS X, from 10.2 to 10.4. With Mac OS X you can set up your computer to log out of your user account when you turn it off which will require a password to log back into the computer. However, if you forgot your user password you can still get back into your Mac computer by using some simple command lines, even if you don't have the Mac Install DVD or CD.
Turn on the iBook G4 and hold down "Command" and "S" on the keyboard as the computer starts up. Keep holding them down until you see text on the screen.
Type "/sbin/fsck –y/sbin/mount –uw /sh /etc/rc" at the first prompt. Press the "Return" key.
Type "passwd [username]" at the next prompt, replacing "Username" with the user name whose password you want to change. Remove the brackets when you type it in. Press "Return."
Enter the new password twice. Hit the "Return" key.
Type "Reboot" and press "Return" when you finish entering the new password for your account.
Enter the new password at the login screen and you will be able to get onto the iBook Mac computer.