In my tests so far it's like 2x8GB ( dual rank ) = max ~3466, 1x8GB ( dual rank ) = max 3866, 2x4GB ( single rank ) = max 3733, 1x4GB ( single rank ) = max 4050.
Everything above ~3600 depends in big part from motherboard and IMC. Motherboard manufacturers are guaranteeing work only up to 3200 while everything above is marked as OC ( even though it was tested, they can't guarantee it will work with all available memory kits ). Also almost all 3466+ memory kits tested and approved so far are based on single rank modules ( 2x4 or 4x4GB ).
I just don't think that most 6600K/6700K and most available motherboards can run stable at 4000+.