gen 4 ssd on gen 3 PCIe - Internal Hardware

What is the difference between PCIe Gen 3 and PCIe Gen 4? can you put a gen 4 ssd in a gen 3 slot PCIe Gen 4 offers a massive upgrade over the traditional PCIe Gen 3 SSD The new standard almost doubled the transfer speeds as compared to cannot determine a partition for slot Rocket 4 ssds are fast, but still not fast enough yet to max out a PCIE Gen4 M2 slot yet Samsung 980 PRO and a couple others similar to it

can you use ddr3 in ddr4 slot My choice of 980 pro for XPS 9500 gen3 is not about speed, it is about almost 10 degrees