DDR4 will be here soon and it puts the axe to dual-channel memory, with DRR4 you can choose any DIMM slot you wish but if the motherboard has 4 DIMM slots then all 4 slots must be filled for DDR4 to work as intended, think of each DIMM slot as an unpopulated lane in a 4-lane hi-way, each DIMM you populate opens another lane on the data hi-way..,