All the non k i5 and i7 can go up 400mhz and is essentially just changing the turbo. Normal turbo speed for your cpu on all cores is 3.6, but will be 4.0 if you set a 42 multi. 4.2 will be for 1 core. It should still be stable without adjusting voltage and the auto setting would probably raise it anyway.
This has nothing to do with safety vs a k and non k. The non k is locked, the k is unlocked, nothing more.