The 4670k has a natural turbo clock at 3.8 GHz, which it normally tends to run at as long the main workload is momentarily on a single thread. What the motherboard might be doing by default is to raise that limit a wee bit so that 3.8 GHz is reached when all four threads/cores are doing some heavy lifting. The CPU's of today can handle that little nudge without any problems what so ever.