Dual-channel memory modules are two memory modules with the same specification, capacity and serial number in the same package.

Because a dual-channel memory is connected to the memory controller through a series of wires, it requires two sets of memory modules with the same capacity and electrical specification. Transfer speed increases as the memory bus capacity increases from 64 bit to 128 bit. Users must install the two modules in the specific DIMM slots to enable dual channel function to have a stable usage.

No, DDR4 memory modules can't be backward compatible with DDR3/DDR2/DDR1. Each module has different pin numbers and notch positions, so they can be installed in repective socket only. The power supply for each generation also varies:
DDR1 DIMMs voltage is 2.5V;
DDR2 DIMM voltage is 1.8V;
DDR3 DIMM voltage is 1.5V (or 1.35V for low-voltage version);
DDR4 DIMM voltage is 1.2V.

Please make sure the memory module is correctly oriented that the notch of the module matches the memory slot, and then gently push the module straight down to the slot.

  *Be aware that you must turn the computer off before installing or removing the modules.