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

And, I also tested the performance gain when connect PCIe gen 4 onto gen3 slot

In ASUS 「PCIe x16 Slots - 4」

pcie expansion slot PCI-e x16 connection or two PCI-e x8 connections The third slot is typically a PCI-e x4 connection This

pcie slot for gpu 2 slots are favored) Do you guys know of any brandsmodels that offer like 5 PCIe slots with the latest AMD