http://overclocking.guide/download/asrock-z170-pro4s-non-k-oc-bios/
This what your looking for?
Multi-core enhancement has been a feature on AsRock and Asus boards on previous intel CPU generations, it allows all cores to boost to the highest boost frequency (3.9ghz in this case).
It's not related to skylake non-k OC, but it's an alternative
The official manual says it's supported but I can't find it.