Sir If we dont consider ur mobo u can overclock it.on
1st idea :4.8ghz
2nd.idea:5.1ghz!
But.u must afford a good water (liquid) cooler.for it if it goes up 4.2ghz so without this cooler dont risk it
Buy a good cooler then overclock it more than 4.2ghz
I said no consider.of ur mobo because I got few information about ur mobo but.I think u will not face any problems with ur mobo about this overclock
Best regards khd