If you're new to overclocking, read the guides right here on TH.com. They are invaluable information for the processor you have. They are easy as could be to overclock but can easily be borked with wrong settings. Once you read the guides to get a good understanding here's what I'd do.
Find a good fixed voltage to run, i.e. 1.05v
Start with IntelBurnTest as a stress test at stock frequency. If it passes two complete runs (20 cycles on stock) then boost the frequency by 100mhz or add 1x to the multiplier.
Then it's rinse and repeat. Once IBT crashes out I'd back off by 1x on the multiplier and run Prime95 for about 4 hours to make sure it's stable.
Remember to watch the temperatures especially when running IBT as IBT will heat that chip hotter than any single thing you'll ever do to it, including Prime95.
If you're not happy with the overclock you have, it's time to add some voltage and start bumping the multiplier again.
My personal limits if I had that chip is this.
75C max temperature with IBT running
1.2v max voltage via Fixed voltage
However others will have different values to guide by. I just rather play it safe. But I think when you read the guides and if you take the advice I have given to you, you should have no problem hitting 4.0Ghz. Just remember every chip is different so noone can tell you if you'll need to add volts at a certain frequency. You have to experiment yourself. You never know, you may have a golden chip that hits 4.6Ghz with very little voltage or you could have a dud that needs tons of voltage just to hit 4.2Ghz.
That my friend is a very nice low voltage overclock. Just remember to make sure to test it and watch the temperatures. Even if the processor seems stable, when it's pushed it may become unstable and need more voltage to handle the clock speed you're at. Running IntelBurnTest will yield max temperatures and find large instabilities while running Prime95 over time (at least 6 hours) will show even small instabilities. So make sure it passes IntelBurnTest with good temperatures and then test it over time with Prime95 to find any problems it's having.
I set IntelBurnTest to maximum and right click the test button to access eXtreme stress mode and run at least 5 passes like that. It takes some time, but has never failed me in finding maximum temperatures and bad instability. After I pass that test with good temperatures I'll run Prime95 for 24 hours and if there's no errors, well I deem it pretty darned stable.