Wow,if thats this PSU then thats a really good quality PSU.Can't go wrong with that.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139010&Tpk=hx750
To unlock the 4th core you usually don't need an aftermarket heatsink but it's always a good idea.Sometimes unlocking the 4th core will require a little more voltage which will create more heat.
The unlocking feature is automatic,should be easy to find in the BIOS.Just turn the Auto Unlock(or whatever it is) On and it should attempt to unlock the 4th core by itself.If it's unsuccesful the firs try,add a lil voltage.About ".025" more to the "CPU Voltage".That should be plenty for the 4th core.
Before you start trying to unlock the 4th core make sure you monitor your temps.Get this application..
Post a screen shot with what temps you see.
EDIT: Just did a little research and it seems the max temp for your processor is 75c,so try to make sure you don't go over that.