The i7-4790K has base 4.0GHz and Turbo 4.4GHz speeds. You're not overclocking, lol, the Turbo will automatically kick in whenever processing load can benefit from it and thermal/power parameters allow some headroom. If your processor is already sucking too much power or running too hot or doesn't really need to bump performance upwards then it just won't go Turbo.
There's no real difference between 4000MHz and 4001.6MHz, such insignificant variations are entirely expected, they fall within the error margins for the sensor/timing/oscillator electricals (or software) components which measure such things.
Even if you did overclock, there's no exact magic threshold beyond which things get risky. The raw number of electrical signals and functions getting gated and channeled through your processor are better viewed as an "analog" population than as anything digitally exact. A part operating at spec clock is already electrically/thermally strained, operated at overspec clock just adds a little more electrical/thermal strain. The only "exact" part of overclocking is the manufacturer's rated spec, which is more of a promise/guarantee that the part will operate at a minimum performance level or it's a defect which qualifies for RMA/replacement.
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