I don't remember and I no longer have my manual. As I recall the lower two are the higher rpm additions and the top two are used 100% of the time. My experience with 2 cycle engines is that the plugs will have oily deposits on them due to the fuel mix. That's one reason Mercury went to the 40,000 volt (open circuit) capacitor discharge ignition system. Reason was to get a fast rising, high voltage pulse to the plug to make it fire before the surface contamination could bleed it off. So don't get all wound up over oil on them, or in the case of surface gap, some tracking across the insulator. Reason for the differences in your plugs is probably the fact that when the second pair kick in you are running 2500 rpms or thereabouts and the engine is warmer and the combustion takes place at a faster and things like that which would contribute to cleaner plugs.
Don't expect your engine to run as smoothly on 2 cylinders as it does on 4. Ain't going to happen as you only have half the power pulses for a given time meaning there is more space between them and it causes more jerking. That's one reason why, back in the '50's I liked the 4 cylinder Mercs over the 2 cylinder OMC engines.
Mark
