Hard to say really I suggest starting at say 1.45v and go up from there....That CPU at 4 will probably take about 1.49v....Make sure that you have proper cooling before attempting that OC and do not exceed 1.55v ever.....To check stability run prime95 small fft's for minimum of 4-8 hours
Voltage settings really varies depending on individual CPU and the motherboard. Try running Prime95 without any overclock and run CPUID-HWMonitor or CPU-Z to check the CPU voltage at maximum load without overclock. Voltage setting in BIOS for this should be AUTO.
Then start doing overclocking step by step (not directly to 4.0GHz) and initially set voltage closest to what you found and see how to behaves with overclocking by running Prime95 for some hours at-least and monitoring system state with HWMonitor (especially the temperatures which should be below 62C I think; but lower is better). If system crashes then try increasing voltage step-by-step (in single units which your BIOS gives you minimum like my BIOS allows +-0.025v at single step). And as lowjack said: don't go above 1.55v.
What is the model of your motherboard ?
For example my CPU 1090T with GA-990FXA-UD3 is at 3.8GHz @ 1.40v (HWMonitor tells 1.41v) with CPU-NB at 2.4GHz @ 1.175v. It all depends on each CPU manufactured and motherboard.