The 930 hits 4.0 pretty easily, my 930 is at 4120MHz stable, had it at 4140 for a few days and it crashed, don't know if that was from OC or from something else so I turned it down.
When overclocking there are two main things to look at: clock and voltage. The clock is how fast the CPU runs. The higher the clock, the faster (and more performance) your CPU (and other components) will be. However, if you push the clock too high your system may become unstable (it will crash with blue screens while running or even at boot-up). On the other hand, voltage has little to do with CPU speed other than that the CPU can execute single instructions faster at higher voltages. Increasing the voltage allows you to push the clock higher without going unstable because of this, but increasing the voltage also increases power consumption (and thus heat output) of the chip. To get a good overclock you need to finely tune the clock and the voltage together and then test to make sure your system is stable and that the temperatures don't get too high.
To adjust the clock on an i7 system, there are a few main things:
BCLK: Base clock, this is the clock frequency of the chipset that is used to calculate the frequency of CPU and RAM as well.
RAM multiplier: This number is multiplied by BCLK to get the RAM speed. Note that DDR ram (of any type) is double data rate, so the real speed is 2*BCLK*RAM multiplier.
CPU multiplier: This number is multiplied by BCLK to get the CPU speed. This number has a locked maximum on most CPU's and is really the only difference between the 920/930/940/950/960 chips. The extreme edition processors (or Black edition for AMD) have an "unlocked multiplier" which allows for extremely high overclocking but only with proper cooling.
In general, you overclock by increasing BCLK. This increases your CPU and RAM speeds together. To get 4.0GHz on an i7 930, I would recommend a 19x multiplier with a 211 BCLK. Since 211x19 = 4009 you're right around 4GHz (a bit over too!) and this is more stable in my experience than 200x20 (exactly 4000) due to unknown reasons. The issue is that pushing your BCLK that high can mess with your RAM (the default BCLK is 133) so you'll likely have to lower the RAM multiplier too. All these settings can be found in the BIOS.
As for voltage, put your CPU VCore at 1.30, CPU QPI at 1.325, DRAM at 1.65 (or 1.66 depending on if your board is even or odd), and IOH/ICH (north/southbridge chips) at 1.16. This is my current settings that work with my particular i7 930. I hope they work for you and if they do work you can try lowering the voltages slightly to see if you can reduce them without being unstable.
To stress test and ensure stability, run either LinX or IntelBurnTest to push your CPU to its limit. These programs deliberately push your CPU so that you can see the maximum load temperatures and test stability. Use CPUID HWMonitor to view the core temperatures and make sure they stay in an acceptable range (I like to stay under 70C but the chip is rated higher, 75-80C may be acceptable but I think it's a bit too high). If any core exceeds 80 for too long stop the test and make sure your heatsink is installed properly and running at full speed. If it runs through one of those programs for several hours without crashing, the system is stable and you can use it at that speed. If it crashes, try turning down BCLK and try again until you find a level where it doesn't crash.