Not too unusual, and in my experience it's caused by a spark jumping to ground and not a carb problem.
A soaked air filter will cause poor running but that's pretty rare, water can't get in the air filter case easily. Pods are open to rain and spray, some cause trouble when wet. K&N are pretty much immune, other brands not so good in rain.
Most likely is the plugs themselves, the insulator should be squeaky clean and the caps should have a tight rubber water seal. The coil ends should be checked as well: the cable insulation can crack right where they go in the coil, you can clean with alcohol and goop over with silicon to seal them. Sparking from cable to tank/frame/whatever shouldn't be exacerbated by moisture but who knows? I think all the fours came with rubber tubes over the actual cables for additional insulation, that's a good idea to add if yours are bare HT cable.
The caps screw into the cable, it's a good idea to check the cable end for decay. There should enough length to trim 1/4" or so off the ends, smear the last 1/2" of the jacket with dielectric grease, then screw the cap back into the core. The cable end of the cap should have a rubber seal as well.
Dielectric grease on the plug cap seals makes them easier to push on, improves the water seal, and stops them from gumming to the cable or plug insulator.
Check your cap resistance. Too much (should be at the rated resistance +/- 10%) can aggravate arcing to metal problems, same for using resistor plugs with resistor caps.
Most of us avoid rain but it isn't so predictable. I have enough to worry about in rain - reduced visibility, obscured visor, reduced traction, slippery greasy roads, and unpredictable disc brakes - to want to deal with a misfiring engine as well.
