Alrighty... Here's what's up!
For 4GHz, you need 210x19 (or 211x19). I'd recommend starting at 200x19 and 1.35V (way more than needed, but a safe buffer). Set the memory to the x8 multiplier for 1600MHz and the Uncore to the 12-16 multiplier for 2400MHz-3200MHz. Though the 16x multiplier is not necessary (Gulftown only requires 1.5x the memory speed for Uncore as opposed to Nehalem's 2x), it is functional and shouldn't be difficult to stabilize. Set the QPI/VTT voltage (or Uncore voltage, I'm not familiar with the SR-2 BIOS) to 1.35V as well. This should be enough to stabilize the uncore. Set the QPI PLL to somewhere around 1.3V, the CPU PLL to ~1.7V, the IOH core to ~1.24, and the memory voltage to 1.65V (or 1.66 if 1.65V is not offered). Leave anything to do with the ICH on auto for now. If there's anything else, it's most likely safe on auto as well.
Set your memory timings manually and you should be good to boot at a stable 3.8GHz.
Once you have this stable, try raising the BCLK 1MHz at a time and booting. SR-2's tend to have trouble with BCLK above 200. Once you've made it to 205, try testing for stability. Then keep going until you get to 210 (for 3.99GHz) or 211 (for 4.009GHz).
The voltages I listed at the beginning should be enough for 4GHz. You can either keep pushing BCLK until you run into instability, or start lowering voltages once you get to where you want to be.
Hope this helps. Let me know if it works!
EDIT: I forgot Gulftown offers two multipliers above the stock one. With that, you could just do 200x20. Silly me forgot and I have one