To set up the ip4000R through wifi, I do confirm that deleting the "installation check" file in the installation package did the work.
The driver itself is indeed present (so that the printer can be connected through USB), but the button "More Printers" in the Advanced section was not present until I reinstalled the package (iP4000R 483.pkg of july 2007 !) after deleting "installation check"
Thanks for the hint !

I am unable to delete the "InstallationCheck" from the installer package. I know there used to be a force delete available but I can't seem to find that. How do I do that?

You can't make changes in the disk image. So you have to open the .dmg file, copy the .pkg to your hd first, then right click and choose "Show package contents".

If it doesn't work, check files and folder permissions ? Do you have an admin account ?