4x8GB is less stress on the MC (memory controller) so DRAM may run ever so little faster, also it can help improve the overall OC on the CPU if doing so
Take a look at the Gigabyte GA-X99-UD4 mo/bo. I put one in my latest build with an Intel i75820K CPU and a Gigabyte GTX 970 GPU. The board will accept up to 64GB's of RAM. I started with 2x8GB's of Crucial Ballistix Sport RAM. This is my best build yet and, everything works great together. If you want to go to 64GB's eventually, you will have to use 8GB stix, otherwise you will end up spending a small fortune replacing lower capacity stix.
4x8GB is less stress on the MC (memory controller) so DRAM may run ever so little faster, also it can help improve the overall OC on the CPU if doing so