Do you have the latest driver for OS X for your Ricoh 4500? There's one dated April 2008 on their website for OS X 10.5.X and posted as "new". Suggest you download and install it. Then try again.
Did you enter User ID and choose "Locked Print" from Job Type? If so then User Code is still grayout, I suspect the device is authenticated (e.g. Windows or LDAP Authentication). When you touch the screen on MP4500, it asks you to enter user/pw? This means your device is authenticated.
If authentication is off, User Code should be available.