Keep in mind that you shouldn't go over 1.35v on QPI/VTT voltage.
Are you running 4.0 GHz with hyper threading on?
Just checked the BIOS, HT is enabled (this was also the case when I ran Prime95 since it opened 8 threads).I don't know if it matters or not, but there is 1 other voltage I tweaked - I set the NB voltage to 1.29v. But as I mentioned, the real breakthrough that allowed me to go from an unstable 3.4GHz to a rock solid 4.0GHz was turning the multi down to 19x and increasing the the QPI/VTT to 1.35v (default of 1.1v and added 0.25v). This allowed me to increase the bclck from 170 to 211 (wihich also juiced my RAM from 1360MHz to 1690MHz)..