With the cooler u should easily get a decent overclock on all cores, but its not the best motherboard so may be restricted on how far u can get, but if u got 4.2 on 4 cores, having that is prob fine as usually only 4 cores are used, but i just never see why to deactivate perfectly fine cores lol
So is the 4.2ghz an oc u got across the 4 cores? if so try a lower oc on six cores unless ur sure u only need the 4