bigbl

hi there,im going to buy a bung for my exhaust so it quietens it down whilst daily driving,i have a jap style back box,cant see a name on it but it has a 5" tailpipe on it,the inside diameter is 114mm,what size of exhaust bung do i get, a 4 1/2" or a 5"? has anyone got a nur spec/blitz or similar they can measure the inside diameter of tailpipe to see what my buying options are? thanks bryan