glow worm 30cxi?

i have a glowworm 30cxi boiler installed in my house ...i had a plummer out yesterday and he said the system was over heating ...but couldnt find a possible cause ...when my heating is on it kicks in runs for a while then cuts out n displays f4 ...i refered to instalation guide ..which told me some possible causes which i adjusted etc but still doing the same ...PLEASE HELP !!