Have Honda Questions? Ask a Honda Repair Expert.
Hello welcome to just answer I am master tech and will be able to assist you.
few questions for you.
has a compression test or cylinder leak down been done yet ?
can you recall the last time the valves were adjusted ?
can you look at the radiator reservoir and see if it's empty or have coolant in it and if so can you smell any type of exhaust or fuel smell ?
the reason for asking these questions is we see on a very regular basis at my dealer is the head gasket fails and uses coolant and will set random misfire codes.
so been this is isolated to the #4 cylinder I would be suspecting a tight valve or fuel injector issue of not a head gasket.
the only injector cleaner that really works is chevron Techron.
I know Costco sells it and so does the chevron gas stations.
knowing this isolated to the #4 cylinder I would first try the additive and also make sure to add this on a empty tank before fill up, make sure to use premium fuel so you get the cleaners in addition to the additive.