Try testing Prime95 at that 4256mhz clock speed but put the voltage to 1.26, lets say. If it stops rebooting it means u didnt have enough voltage, if it keeps rebooting then it could either be a RAM problem (try to force the ram multiplier under at x8 or x10) or simply a problem issued by your motherboard's wrong voltages (which would mean that ur mobo doesnt manage to OC properly because its defective.) You know... when you OC ur CPU some things in ur motherboard are also getting clocked higher.
And it is indeed weird that its simply a reboot happening and nothing more. Did you check you motherboard's temperature when your CPU is OC'ed and running prime95? You can do that with "CPUID HWMonitor" program my mobo is at max 50C when my cpu is at stock, but when i OC it, my mobo goes up to 65-66. If it goes much higher than this is could be an issue that makes ur mobo order a shutdown to stop the heating, because high temps there will not cause BSOD in windows.