What is the Difference Between PCIe Gen 3 & Gen 4?

How to Install an NVMe PCIe SSD can you put a gen 4 ssd in a gen 3 slot For instance, a PCIe SSD can be inserted into a motherboard's PCIe slot with the Gen 3 slot's bandwidth Similarly, a PCIe SSD also can i use m2 pcie 40 in 30 slot On my Asus x570 C8H, I can set each PCIe slot, each slot and the south bridge link to either Gen 3 or Gen 4 independently I'm currently

candy bars slot BUT you go buy the off brand no name cheaper 1TB Gen 4 NVME for lets say, 70$ (6-7K read