If you're consistently misfiring on one cylinder, that means you only have 3 to power the whole car. Your gas mileage and power should both go down by about 25%.
Yeah, I agree. If there's anything like a distributor, you're describing perfectly the symptoms of water having gotten into it. Blow it out with some compressed air.
Otherwise, if it's only that one cylinder, maybe the coil pack for it (if they're individual) is bad, or the connection got wet and corroded.
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