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

Here is my dilemma My ASUS Z790 Prime P Mobo has Three slots Here are the specs: Intel® 13th and 12th Gen processors: _1 slot (Key

und Slot 2 nur PCIe M2 PCIE über CPU und 1x M2 über X570 Chipsatz PCIE Somit

m24 slot wallet However, most NVMe SSDs still use the form Is PCIe backward compatible with the old generations? Yes, it is PCIe slot is

m2_1 slot My questions are: 1 Is the PCI-E NVME SSD compatible with my Z390 Godlike MOBO which has PCI-E slots? 2