There's an article on tom's that concludes that DDR3 above 1600 MHz provides diminishing returns. 1600 is the sweet spot, it seems. Maybe until latency on higher clocked memory improves. And what overclock settings are you going to apply?
If you go to 4.0 GHz, you'll end up with a base clock of 200 MHz. Then you set your memory multiplier to 8 (8 x 200 = 1600), set the RAM timings as per manufacturer spec, up the uncore voltage to no more than about 1.3 V (try lower voltage first, though) and DRAM voltage to no more than 1.65 volts.