M.2 PCIe SSD are not like RAM where the faster RAM runs at the slower RAM speed. As long as RAID is not set then the drives should run at their dedicated speed. That is gen 3 at gen 3 speed and gen 4 at gen 4 speed.
I currently have the XPS 9700 with two SSDs (both are PCIe gen3), the original SSD and a Samsung 970 EVO Plus 2TB. The EVO is just a storage drive, and the OS is installed on the other drive. I just ordered the XPS 17 9720, with a 1TB SSD. I understand that the new SSD is PCIe gen 4. Ideally, I would like to add the Samsung 970 EVO Plus as a 2nd SSD in the open slot. However, I'm not sure if installing a PCIe Gen 3 will slow down the other SDD, which is PCIe gen 4. I would assume that it would only slow both drives to PCIe gen 3 speeds if both drives are set it up in RAID. But, if I switch to AHCI, will I be able to run both PCIe Gen 4 and Gen 3 speeds?
XPS 17 9720 - Can I mix SSD PCIe versions?