Hmm ok. Try using 1.3725 vcore, 1.22 Vtt, 0.67x GTL Ref, 1.55 vNB at 455 FSB. Also try advancing you ram skews to around 250 - 300ps if you have them.

Also, can you raise GTL Ref above 0.67x? I have seen some guys with C0 stepping 9550's needing around the 0.68x - 0.73x mark for 4GHz, but some also need around 1.42v load for stability at 4GHz.