It is not a case of a "best voltage" for any particular frequency. You see, all CPU's do not operate at the same voltage.
Intel's max recommended voltage for the 45 nm is 1.45 volts. If you can reach 4.0 GHz on less than 1.45 volts with your particular chip, great. OTOH, you may not be able to reach 4.0 GHz at any reasonable voltage.
Overclocking is always a case of "YMMV".
on my pc I got an i7 930 stable overclock of 4ghz at 1.29 volts however its like the other guy said, it may depend on which batch you have got and all cpu's have a different stable rate at different voltages. I know other people who have got a really good batch and can use 1.25 volts and some people who were unlucky and have to use like 1.38 volts, If I were you I would try 1.3 volts to start off with and see if it's stable, if it is try go lower with the voltage, if it isn't go higher until it is.