the misfire, pull cyl 4 plug check it out. you may just need a tune up. if over a 100,000 miles. or a plug wire may have burnt to a manifold.
coils crack too you can swap them around to see if the mis follows to the new cylinder.
as far as the stalling. has it started again at all? check for fuel and spark are steps one and two. if those are good and its running again all by its self, the crank sensor could be the problem, they never set a code, and they stall sometimes fire right back up, sometimes a week later it will like magic fire up.
the elbows, you seem to have covered. just clean the 4 holes out real well where the elbows go, lube the new o rings with oil and slide them in.
