The spark strength is not effected by the heat range of the plug. That only controls how well the heat is transferred from the ceramic to the metal portion of the plug. If the plug gets too hot, it can cause pre-ignition, and it can damage the internal resistor, if it is a resistor plug, which those are. So you select the heat range so that the plug operates at the correct temperature. Cold weather causes the plug to cool off a little too well, so changing to a plug that doesn't transfer heat as well, puts the plug temp back in spec. Conversely, running at extended high speeds tends to overheat the plug, so selecting one that can move that heat into the head better will keep the business end of the plug from getting too hot.
Put the stock plugs in, run them a while then pull them out and look at them to see how things are running. Who knows why you had a mix of plugs. Might have been what was on hand, or was done for a reason, like you can run a hotter plug on a cyl. with high oil consumption to help keep the plug from fouling as often. Not a fix but more of a band-aid. No idea why you would have two cyl. with colder plugs, at least not on a water cooled engine.
My best guess as to why two of the plugs were the wrong ones (high RPM plugs) was the previous owner. He isn't the brightest when it comes to proper maintenance of anything....lol Tractors, trucks, cars, and bikes; if it's a machine, it will likely be neglected. Just today I told him that it was important to change the nickel plated plugs earlier than normal, roughly 30,000 miles give or take. I'm told that these plugs are not known for lasting very long. He laughed and said, "Not my Goldwing! I can go forever on those plugs and mine doesn't have nickel plated plugs". I'm like, "huh? Are you sure? Could that be the reason why your bike doesn't start very well and tends to stall whether cold or hot?" LOL He's the one that sold me my Goldwing because he couldn't figure out why it wouldn't run right. So he sold it to me and purchased another one....a 1998 Goldwing A. It smokes (blue) bad when starting up and runs like hell. Once it warms up, it runs better, but after shutdown and restart, it spits, pugs, and sounds ragged, but he thinks everything is fine. LOL
My wife's Dodge RAM peace of crap Hemi just went through that. It misfired bad on two cylinders and I thought it might have been the typical Hemi valve seat failure. Turns out the nickel plated plugs have to be replaced every 30,000 miles according to Dodge specs; at least that was what I was told by the service department. What is the recommended plug replacement frequency of the Goldwing?
Right now I'm running the correct spark plugs on all cylinders, NKG DPR7EA-9 standard plugs, and she runs fairly well. Some of my issues involves tuning and syncing the carbs, and also resetting the floats in the carbs to 8mm instead of the 7mm I mistakenly set them at last year. I will do all of this next month when I have all available parts, i.e. factory muffler with baffles, new plugs wires, yank the carbs for another rebuild and resetting of the floats to 8mm, and then I'll perform a proper idle/air mixture adjustment and carb-sync. Should be perfect by then. I suspect also that the idiotic free-flowing sawed exhaust (sawed as in jury-rigged) may be contributing to engine performance issues. As a test run, I removed the chrome jury-rigged caps the previous owner installed on the mufflers, and I wrapped it with baffle wrap (fiber glass) and re-installed the chrome end-caps; I hate those ugly things and it makes the Goldwing sound hideous with that absurdly loud over-toned bass humming. I noticed improvement to engine performance on the low-end since a little more back-pressure is supplied to the exhaust system. I know from experience that modern machines must have some form of back-pressure which helps improve fuel efficiency, helps keep the exhaust valves a little cleaner, and reduces exhaust flow turbulence. It reduced the over tone about 10% percent but still annoying. I cannot wait to get the factory mufflers....this jury-rigged crap is driving me insane. Ear drums want to vibrate out of my head.