Hi
Just recently had a Baxi boiler not working,turned out to be the Air diaphragm that senses the Fan running was faulty,I thought I had found the problem at first,the micro switch controlled by the diaphragm moved on it's fixing rivets!,superglued the switch in the correct position,
turned out the diaphragm was at fault aswell.
Sorry off topic a bit but may help fix your boiler,faults usually sensors and valves etc more than electronics.
4 pin I.C does sound like an opto isolator/coupler.