What is the difference between PCIe Gen 3 and PCIe Gen 4?

4 x Motherboard nvme gen 4 in gen 3 slot BUT you go buy the off brand no name cheaper 1TB Gen 4 NVME for lets say, 70$ (6-7K read nvme slot types The card physically has an X4 interface, so it could be plugged into an X4, X8, or X16 slot It can still physically only access 4 lanes (else

nvme in pcie slot This is running off a chipset fed PCIe NVME slot on the X570 Aorus Xtreme, I understand it's not going to be as fast as a CPU provided NVME