dean wrote:
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?

I've seen a lot of recommendations for various drills, but what about
marking the hole and using a rotary zip saw to cut it? I'm sure there
are masonry bits available for those larger zip saws, considering even
my little Dremel tool has such a thing for small masonry cuts.