? about 07 Elantra SE

I need some advice about my 07 Elantra SE (auto trans). I have owned the car since November 06, and the typical mileage (almost all city) has been about 27-28 mpg (in other words, about what I'd expect).
About four weeks ago, I got a yellow indicator lamp and brought the car in to the dealer. Here are the codes from the printout I got from the dealer: P0304 intermittent misfire in cylinder #4 spark plug and P0442 small evaporative leak. The fuel door was loosened and then tightened, and the cylinder #4 spark plugs replaced (again, I'm reading from the printout).
As far as I can tell, the car has performed the same as usual, but the trip computer shows that the mileage has gone down to below 25 mpg, for the exact same driving I've always done. Does the drop in mileage indicate that the spark plug misfire wasn't properly fixed? Thanks in advance for any help you can give me.
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Everything is probably okay. Keep in mind that the trip computer displays an average MPG. If it hasn't been reset since prior to the repair, it's likely to be counting data from when the car wasn't running properly and from when it was idling in the shop.
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