My 1993 MX-6 idles rough sometimes, and other times purrs like a kitten. Twice within the past week, it has stalled at a traffic light and refuses to start until it sits for a little while. The front cylinder (4 cylinder) has a new plug, is getting a spark, but is the one that is missing. It has had the injectors cleaned recently with a new fuel filter, but it didn’t help the idle. If it were an older car, I would say “vapor lock”. Could it be a faulty fuel pump, trash in the line, or what. The car has 180,000 miles on it.
How did you determine that #4 is missing? Did you pull the spark plug wire, while the engine was running, and note the rpm drop? Or, did you use another method? There could be lower compression in the cylinder, or a valve sticking on # 4 cylinder. To check, do a compression check and a vacuum TEST. The full page (or, two) instructions on how to do a vacuum test are in any Haynes, or Chilton’s repair manual.
With the engine running, I pulled the plug wire. The engine continued to miss like it had with the wire on - no loss of RPM. Also the wire sparked when close to the block, so I knew the spark was getting to the plug.