NVME gen 4 or M.2 gen 3 NVME?

just got a new NEO 5 with M.2.NVME case and wondering which NVME do I need to purchase. NVME gen 4 or M.2 NVME gen 3 PCI-e? either should work or only M.2 NVME only? thanks

i used a kingston nv2 pcie 4.0 nvme M.2 1TB and it works fine. hope it helps