I suspect I have a bad ignition coil: engine suddenly running very rough (120,000 miles), feels like only 3 cylinders firing. I want to use an ohmmeter to test the coils, but I don't know which of the 4 electrical contacts are the primary wiring and which are the secondary. Anyone know?

 

Have you scanned the vehicle for codes? If you do indeed have a misfire, the codes should tell you which cylinder. Simply swap the coil pack between the offending cylinder and the one next to it and see if the misfire follows it by re-scanning it.