Is DDR4 RAM compatible in a DDR3 RAM slot?

No They don't even fit in the same slots There were a very few motherboards that had DDR4 and DDR3 slots, but you could only use one

More tutorials: Let us present a how-to guide for DDR3 memory

can you put a pcie x4 in a x16 slot Every DDR generation is different from the others DDR4 is not backward-compatible with DDR3 so a DDR4 DIMM will not fit on a DDR3 DIMM slot

can you use pcie 40 in 50 slot That's doubly true now that Intel's 12th- and 13th-gen motherboards can use either DDR4 or DDR5 RAM, and AMD's Ryzen 7000 processors made the