According to Stormin Mormon :
Many gennies have circuit breakers for this situation. I doubt a 4 KW would
power the grid (down to a lineman) without tripping off the breaker. But why
take chances?

Indeed.

Let's say you're the only person fed off a particular 4K_volt_ pole pig, and an
ice storm has pulled the supply wire off.

You've inadequately set up your generator, and the generator is back feeding
the pole pig. The input side of the pole pig is now presenting 4K _volts_ to
an unsuspecting lineman. But that's the dead side, right? Wrong. _Both_
ends of the break are live.

Expecting a 40 _amp_ breaker to trip fast enough (or at all) to save someone's
life? Not a chance.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.