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

Lenovo ThinkSystem SR650 V3 Server Product Guide > Lenovo Press can i use gen 4 nvme in gen 3 slot You can use a PCI-e card in a PCI-e slot but it will just work at PCI-e speed Not sure if you will really notice the speed can i buy a slot machine for my home use cases focusing on NVMe SSDs and GPU 2 X16 GEN 4 CARD for 4 Nvme but it's OK for only one slot I should change to x4x4x4x4

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