Hi Stellarweb,

I've been reading through what you guys have been doing to try and solve your problem. I've of the mind that we should take a phpMyAdmin full sql backup and try to restore that into another site's testing database. This should tell us if we have:
1. code related issue
2. database related issue
3. server related issue.

not everything would be perfect for this test since you might have other mods installed, but it may help to diagnose your issue.

Did you attempt to install zencart in a new sub-directory, install only EP4 and test?