If you don't have a direct comparison, you shouldn't notice that the Samsung 980 Pro is "slowed down" with PCIe 3.0.

https://www.igorslab.de/nvme-ssds-...

Samsung Electronics GmbH
Hello,
Thank you for your enquiry.
You can also operate the Samsung SSD 980 PRO via a PCIe 3.0 interface. However, the maximum performance is only achieved via a PCIe 4.0 interface. Put simply, this is because PCIe 4.0 has twice the throughput of PCIe 3.0.
Best regards from Schwalbach,
Your Samsung Service Team

Yes, it is also compatible with PCIe 3.0. But it cannot use the full speed. The M.2 is fast enough even with PCIe 3.0.