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16 mpg on a misfire...I'd be happy to see that on my truck! But ultimately, a misfire will absolutely affect mileage. Since cylinder #4 (the "04" in P0304) is easily accessible on the engine (driver's side), you could at least swap the #2 and #4 coil packs to see if the problem travels, and pull the #4 spark plug to check its condition and replace if warranted. Those two actions would at least rule out spark issues. Air and fuel you can't really check without tearing down the top of the engine a bit, but maybe a borescope might show something. In tracking down my own engine issues in recent years, a smoke machine proved super valuable in finding leaks. Maybe run a fuel system treatment for a tank or two in case it's something with the injector. Unlikely the code is tied to clogged cat. Two other cylinders exist on the engine bank, so you'd throw a bank-specific cat or O2 sensor code instead.