DDR4 memory differs from DDR3 in memory density, clock speed and voltage.
Current mainboards mostly use DDR4.

These are different standards. DDR4 is newer and faster than DDR3 and is used by all new mainboards. DDR3 RAM cannot be used on a DDR4 mainboard, and vice versa. I don't have the exact specifications of DDR3/DD4 in my head, but you can read up on them if you're interested.