What is the voltage variance thats good (bad) appropriate for an Intel Pentium 4 processor?
That would really depend on which Pentium 4 you’re talking about exacty. Numerous processor cores have been given the “Pentium 4” name over the years, all at varying stock speeds and voltages. I think most of them run at around 1.5 volts, give or take a fraction depending on the specific type. Core voltage can be modified from stock, though the specific results can depend on your specific chip and the various manufacturing imperfections it has. Generally, voltage has to be increased when overclocking to achieve stable performance, but as voltage is increased power consumption and therefore heat is also increased, which can degrade stability as well, or in some cases lead to permanent damage. More specific information would require a less vague question. 
Intel will have operational specs on any of your P4 processors. They will have a program to determine what processor you are running, if you don’t know.