Can a PCIe graphics card work on a PCIe slot?

Can a PCIe graphics card work on a PCIe slot? can i put a pcie 4 0 in a 3 0 slot it will be fully compatible on PCIe as long as it is the same form factor Back in the days people used to buy PCIe gfx and put into candy candy slot PCIe slots and still can't make them both use x16 Using GPU-Z, I found the following: - GPU-1 : supports x16 , using x4 -

can you use ddr3 in ddr4 slot It should also be noted that PCIe is backwards compatible You can put a card in a slot, or a card into a 3