According to dean :
Ok I know i can drill a whole load of little holes in a circle and then
knock it out, but is there such a thing as a 4" drill bit that will do
this more easily?

As others have suggested, renting a core drill, or using a hammer
and appropriate chisel are the simplest way.

I have, however, watched experienced electricians punch 2"
conduit holes in cinderblock with nothing more than a 2
pound hand sledge. Not perfectly circular of course, but
close, and you have to seal it anyway.

I probably shouldn't recommend that tho, unless you have something
to practise on ;-)
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.