4 x Motherboard

What is the difference between PCIe Gen 3 and PCIe Gen 4? can i use gen 4 ssd on gen 3 slot PCIe Generation All PCIe Generations slots and slots are backward compatible so there is no need to worry about purchasing the wrong expansion card or NVMe can i use pcie 30 in 20 slot On the it says PCIESATA does that mean I cant put gen 4 there? At the moment I have a WD PC SN530 PCIe Gen3 x4 NVMe installed on slot

can i put a pcie 30 in a 40 slot yes, it should depending on the ssd but if it's nvme make sure that the motherboard supports it