What does the X4 mean in an PCIe Gen X4 NVME drive?

You're referring to NVME based M2 SSDs, correct? But yes if the SSD is PCIE spec then of course the speed will be less when used on PCIE

m 2 9-2 937-0 1 30 1 6 243 9 24-1 1 55 95 11 1 58'4 9 8-1 12 40 0 16 slot , having thin edges at the bottom , but so recessed above as to

m2 slot screw For our lines of high-speed PCIe® NVMe® SSDs, the Crucial System Scanner and Crucial System Advisor will list all 1 GBs 2 GBs 4 GBs

m2 expansion slot is a slot that can interface with SATA , PCI Express