Based on nVIDIA chipset specification, the motherboard you have only support 3 DIMM slots officially. When a MB comes with GIGABYTE unique memory H/W deisgn, the board you have can provide the extra DIMM slot and make users get better flexibility on H/W installation/expansion. But, because the quality of memory modules in retail market is uneven, thus, when you install 4xDDR400 memory modules on the motherboard, our MB BIOS will decrease memory speed to make sure most system will bootup properly.