You said in the bios it sees only 3 cores right? First of all i think it is a bios problem, it doesnt detect the new cpu how it should. You got 3 options, buy a z270 motehrboard, return it and change it with a 6700k or wait for a bios update. Also you can try to reset your windows but i would not hope much because you did not change the motherboard so it should detect the cpu without any issues.