The multiple misfire code might be telling you something about a general condition that exhibits itself most often in number 4 due to tolerance stacking and/or maybe two separate issues.
Does your mechanic friend (or you) have access to a scan tool where he/you might look at misfire counts, watch the numbers in real time, check O2 sensors, fuel trim, etc
Although a long shot given the inspections that have already been performed, I probably would do a quick and dirty compression check on 4 as well.
I have borrowed a code reader and will see if it does that