you can easily overclock i5-6400 to 4.05GHz (this is what I have now), you need to have motherboard bios version with skylake non-k overclock ability, I think all motherboard manufacturers offered official bios with that ability, obviously later on under pressure from intel they removed the ability in later versions of their Bios, but it's still available definitely for Asrock
when you update your motherboard with correct bios just set up bclk to 150, cpu vcore voltage to 1.2V, keep Ram up to 2600MHz and that's it , job done