It would be much cheaper for me to custom build a more powerful system. I could do it for half the price of a 7875.  But many Dell Precision workstations, or servers, come up on ebay for cheap. 

I found a list where the question is answered:

https://en.wikipedia.org/wiki/Dell_Precision

Just get a Precision 7875 for futureproofing of the next couple years.  It offers Gen 5 PCIe x16 slot for your graphics needs.  Unless someone compiled and updated the list in Wiki, Dell only provides the technical specifications separately for each systems.

As I mentioned, you could check out Wiki.  While it's not complete and up to date, you should be able to find the information for the systems you have in mind.  Dell only provides technical specs for each released system.  You can use that as well.

Have you considered reaching out to Dell's customer support or sales team to inquire about PCI Express 4.0 compatibility for specific Precision workstation models?

OP already followed the advise and found the answer from Wiki list.

Dell Precision workstations that support PCIe 4.0 include the Precision 3650, 3680, 7875, 7560, and 7760. These models offer faster data transfer speeds and are equipped with PCIe 4.0 x16 slots. Make sure to check the specific configurations, as performance gains depend on the hardware used.