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Interesting. You can select that option and it may provide you that speed. It may also prove to be unstable but you should be able to restore default speeds if it doesn't work. I would monitor temps real good especially if using the stock intel cooler that setting.
Be Aware though: Overclocking does void your warranty since 4.2 is well out of Intel's specs for that CPU
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.