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

PCIe slot shenanigans pcie 3 0 x4 slot The Intel X550-T1 specification states it uses a PCIe x4 slot, however in my new PC , I'd like to use it in the PCIe x4 pcie m2 slot PCIe boards can fit into slots designed for their lane configuration or higher Plugging a x4 PCIe into a x16 slot is acceptable

pcie 40 in 30 slot Slot 2, PCI Express x16 ; Slot 3, Slot not available on this board ; Slot 4, Slot not available on this board ; Slot 5, PCI Express x8 in a