Can't overclock that CPU on that motherboard. You would need the 6700k. To answer your question, 4.4 is a modest overclock. A cryorig H7 air cooler would handle that easily.https://pcpartpicker.com/product/93Crxr/cryorig-cpu-cooler-h7
Pretty sure the Z170m-Plus does in fact allow BCLK overclocking on factory UEFI, it certainly does on non factory UEFIs.
I'm no longer sure which Z170 motherboards still allow bclck since Intel has updated the microcode. Some boards are still able to I know, but others with newer Bios can't be reverted to the older Bios, so I typically recommend one of Asrocks Hyper series motherboards if someone is interested in overclocking locked processors.