To tell you up front... I am a complete and total overclocking newb.  My current 3.8GHz OC was mainly achieved by using specs from another source and some minor tweaking by forums users here.  I plugged the settings in and it worked automagically.
 
Hardware
CPU: Intel Xeon X3440
MB: EVGA P55 FTW
Cooling: Watercooling loop
 
Settings
Bus Speed: 200
Multiplier: 19
VCORE: 1.325
PCH Voltage: 1.05
CPU PLL VCORE: 1.8
CPU VTT Voltage: 1.225
DRAM Voltage: 1.55
VDroop: Off
Turbo: Off
 
This config has given me a rock solid 3.8GHz OC that has let me fold numerous bigadv WUs without issue.  But... I want more... I want 4.0Ghz!  I know this CPU can do it, I just don't know what I should set to get there.
 
I tried a few things: set Bus Speed to 211, kept the 19 Multiplier, upped VCORE and VTT a little bit, but the system would lock up as soon as it started folding.
 
Thanks in advance for your help.  Just let me know what I should set.

I was able to bump up to 3.9xxGHz stable, but the 4.0GHz OC continues to evade me.
 
Actually, I am amazed nobody has given any suggestions yet.

Is 19 the max multiplier?
If not then try 20 Multi. and Bclock 205.
Then adjust voltage as needed.
 
I don't think your hitting the wall, just a power bclock thing!

Using turbo will allow me to use the 20x multiplier, but that only affects one core (not good for folding), so I'm using the 19x.
 
I've been able to run stable with 3.914GHz (19x206) with VCORE at 1.3xx and VTT at 1.4V but whenever I go up, the PC either locks or the linux kernel panics and reboots.

 Vtt is for uncore so you should not have to up it as high as 1.4. Lots of people use vtt as a crutch, does not need to be 1.4. Too high can cause problems also. Are there no other settings for voltage other than the ones you have listed? Also do you have an uncore setting? If so set it to auto. What is your memory specs? Is the memory being overclocked?
 If you can list a template of all your settings to do with overclocking not just the few you have listed. Unless of course that is all you have.( if those are all you have then the problem may be the board, it was not really designed for overclocking.) More specs like what memory, board, psu, and what ever. If you want real help.
  I may have said I don't have a lot of experience with 1156. But I sure as heck can OC.

OK, here are the BIOS settings (I took pics because I'm lazy right now):
 
RAM...

 
CPU...

RAM is rated at 1.5V but I had to raise it to 1.55V for stability even before I started to overclock.
 
I'll try those settings out.
EDIT:
 
Set PLL to 1.675V because there was no setting for 1.7V, next step higher is 1.8V.
 
The RAM is DDR3-1600 so setting it to 1333 would end up setting it over 2000, I left it at Auto and manually set it to 1066 using XMP and Standard profiles.  The system locks up as soon as it starts to fold. 

Trying it out... XMP profile, Frequency set to 800 MHz (options are Auto, 800 MHz, 1067 MHz, 1333 Mhz).
EDIT:  Fail.  Would not boot.  Had to clear CMOS.

I have 4 sticks of GSkill RAM.  I put this folding rig together on a budget, so it's very possible that I have reached the limit of what it can do.

 With 4 sticks vtt may need to be a bit higher.