Here are actual measurements of 48 No4 boltheads.
#0---11 examples---.627-.632".
#1---17 examples---.630-.634". 1 example---.626".
#2---10 examples---.633-.638".
#3---8 examples---.635-.641". 1 example---.625".

Most of these are on rifles---with the exception of the .625' #3. It's a brand new BSA made piece and probably "new" because it was so far out of spec.
At any rate, I suspect this is a representative sample.
-----krinko