Being a retired IT professional, and therefore, by definition, a little OCD, I did a little digging and have come up with the following information and theory. Nowhere is it specified that only one SSD drive is capable of running at GEN 4 speed. 
Recently bought a 7560 Precision work station.  Based on the Customization page, the only SSD drives they offer are PCIe Gen 4 drives.  In the Service Manual for the 7560, on page 16, where it shows how to remove the secondary M.2 SSD, it states ": This slot supports only M.2 2280 PCIe Gen4 Solid-state drive and does not support any Optane, or M.2 2230Solid-state drive."In the Precision 7560 Technical Guidebook, on page 39, you'll find specifications for a 1 TB  PCIe Gen 4 SSDWhen I installed (2) Samsung 980 PRO drives in the available slots on the motherboard, they ran at PCIe Gen 3 speeds.Samsung Magician could not report the Interface nor could it figure out the driver in use for the new drivesI changes the boot mode from RAID to AHCI  (no small feat - see other threads about how to do that) Now Samsung Magician is reporting the interface is PCIe Gen 3 and the driver is "Microsoft"Samsung does not have a driver specifically for the 980 PRO and if I try to load the driver they DO have, I get an error, "Samsung NVM Express Device is not connected."Tech support insists the slots are PCIe Gen 3 and that seems to be the case.  Why then, would all the Dell documentation I can find indicate that you CAN run gen 4 drives AND the only drives they offer to customers on their customization page are gen 4 - that will only run at gen 3 speed?Has anyone actually gotten gen 4 speeds out of the secondary SSD slots?  The Primary slot for the OS drive does run at gen 4
Why doesn't my 7560 run Samsung 980 PRO SSD's at full speed?