DDR4 vs DDR5: They are physically and electrically incompatible — pick one and stick with it

DDR4 and DDR5 cannot be used in the same system. The slots look similar but have different notch positions, and forcing a DDR5 stick into a DDR4 slot would fry something. Even motherboards with both slot types only allow populating one set at a time — it’s “DDR4 or DDR5,” not “and.” The memory controller on the CPU is designed for one or the other, so mixing isn’t an option.