Sorry, realizing I misread or misinterpreted the initial post. I think the assessment of the problem is wrong, but not being there in person to see first hand and actually test these locos and your Z4000.
The point is, I remember distinctly when you got the Z4000 and followed these topics. I was not a forum member during that time so I could not reply.
https://ogrforum.ogaugerr.com/...7#161349519396080797
also here https://ogrforum.ogaugerr.com/...1#159237712048002711
What is the serial number on your Z4000?