It took me a lot of fine tuning to get my X3 440 stable at 4Ghz - and I reckon I've still got tweaking to do. But I have got 4 cores (unlocked) stable.
I had a locked multipler, so was forced to go the FSB route too. - Now the trouble with this, is it overclocks the CPU NB, the RAM and the HT link. What I had to do was reduce the HT link multiplier to keep the HT link speed at around 2000Mhz - up to 2200Mhz was ok for me. Second, I had to change the RAM multiplier - to keep it within the RAM's capability.
I'm running 267Mhz FSB, 2136Mhz HT (I had to drop the multiplier from x10 to x8), My CPU-NB runs at 2670Mhz (still on x10 multi) and the RAM I had to drop the multi on. My Ram can do 1866, ran at 1600 on default (800Mhz in the BIOS, (4 X 200)) - But to keep it within spec, I had to drop the ram to 667Mhz ( ~3.33 X 200). the reason for this is easy once you do the sums -
at 267 FSB, at the 800 setting my ram would be 4 X FSB = 4 x 267 = 1068Mhz - clock doubled, that 2136Mhz - way over my RAMs speed rating of 1866Mhz.
Dropping the ram to the 667Mhz in the BIOS -
at 267 FSB , 667 setting, the RAM would be roughly 3.33 X FSB = 3.33 X 267 = 889.1Mhz - clock doubled is 1778.2 Mhz - Under my 1866Mhz ceiling.
Knowing my RAM is under it's max speed, means I can pretty much eliminate that as my OC failure problem - That allows me to concentrate on the CPU speed, CPU-NB speed, and the voltages.
So do your sums for the ram speed, and keep it under its stock capability - it can be tweaked once the CPU OC is stable.
I would start stock, and jump 10Mhz at a time to start with, and once your HT link gets up to about 2200Mhz, drop its multiplier as you increase the OC. Once it start failing, raise your Vcore (use CPUz to keep an eye on Voltages - I found mine were higher than set in the BIOS, changing my LLC % eventually brought it into range - I had to set LLC lower (16-odd %))
it should let you squeeze a little more - then once you hit the wall again, raise your CPU-NB voltage a bit, and see if it stabilises.
if not a little more Vcore.
I'm on a deneb core - my Vcore can go up to 1.55V -I tried to stay about 1.5V for my limit. My CPU-NB voltage could go to 1.4V, but 1.3V was enough to keep me stable. Find the safe 24/7 limits for your chip Vcore, and CPU-NB Voltage, this will give you your safe limits to try. And keep an eye on temps - more volts and speed = more heat.
I found I got to 3.8Ghz - then my voltage requirements increased dramatically to get stable at 4Ghz. to get that extra 200Mhz required Vcore from 1.4V to 1.4875V (and it peaks at 1.52 in CPUz). My CPU NB voltage had to go from 1.2V to 1.3V to get stable too - and showed symptoms of requiring more Vcore, when the CPU-NB was to blame.
I hope you can glean something from my experiences to help you get over 3.8Ghz - it does take a fair bit of fine tuning. LLC was the key for me too, it helped stabilise it.
you can try OC the RAM for more speed once you know the CPU is stable.
Best of luck.
(and apologies if i went OTT