is an overclock of your i5-760 thats pretty detailed. 4Ghz sounds pretty average for that CPU. Almost any processor can achieve a minimum of 20% overclock.
As others have said, dont adjust the voltage. When you increase voltage you start decreasing the life expectancy of the CPU. When you increase the processor frequency, run "Prime95" (program) stress test for 20 minutes to make sure you have a relatively stable overclock. If its stable, take another step up, repeat, repeat, repeat, until Prime 95 returns errors. Then back off a bit and run Prime 95 overnight. If you wake up and your computer is still chugging along you've got a stable overclock. Watch your temps while you're running Prime95 as it will load all cores to 100% usage (creating the most heat you will ever see from your CPU). If you are running on a stock CPU heatsink, your overclock will not be able to go as high as with an aftermarket cooler or liquid cooling option. I dont like to see my CPU temp go over 70C on Prime95 (less is better) so my overclocks never get to the theoretical maximum but it gives me some peace of mind knowing that I'll never fry anything. Keep in mind that 70C is after a good 20 minutes of Prime95 running all threads/cores at 100% usage which will rarely be seen for that duration under typical usage. If its an overclock for everyday usage I dont like to see more than about 60-65C with Prime95 running
i5-3570K, ASRock Z77 Pro4-m, Asus GTX660 TOP, 120 GB Vertex 3 Max IOPS, 2 TB Samsung EcoGreen F4, 8GB G-Skill @1.25V, Silverstone PS07B