turbo core disables half the cores and ups the clock speed, so it stays stable under stock settings and temp with stock cooling, if it just jumped in clock speed it could be unstable and or get too hot for the stock cooler, so its not programed to do that.
Turbo core is a feature built into the cpu to activate when needed, its not an overclock setting, that cooler should do a good job at overclocking all cores, where there is no needed for turbo core. So what cpu do u have? along with other specs.
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