are ddr2 and ddr4 compatible?
Is DDR4 backward compatible with DDR3, DDR2 and DDR1? All DDR memories are mutually incompatible. ... The power supply for each generation also varies: for DDR1 DIMMs, it's 2.5V; DDR2, 1.8V; DDR3, 1.5V; and DDR4, 1.2V.