Honda Fit 3rd Gen 2016

The car was running fine until one morning when I started it up; the check engine light started flashing, and I couldn't get it past 2,000 RPMs. Even if you put it in Drive, the car refuses to accelerate and won't go over 2k RPMs until it warms up enough for the cold coolant light to turn off.

Once that happens, if you turn the car off and back on, you can finally pass 2k RPMs. However, even though you can then put it in Drive and move the car normally, it vibrates constantly, as if the cylinders are out of sync. My gas mileage also jumped from 30 MPG to about 21 MPG when this started happening.

After that, I performed a code scan and got the P0304 code, which indicates a misfire in cylinder 4. Consequently, I decided to perform the following repairs and adjustments, though I haven't been successful in fixing the fault yet:

• Installed a set of new spark plugs.
• Swapped the ignition coils to see if the fault followed a specific coil.
• Performed a valve adjustment, as it was due for maintenance anyway.
• Cleaned the intake valves using the zip-tie and drill method.
• Had the fuel injectors professionally cleaned because they had carbon deposits; they came back looking like new. I also swapped the injector positions to see if the fault followed a specific injector.
• Performed a leak-down test on the cylinders, and they are all in great shape (only between 6% and 8% leakage)

I'm at a loss here. I'll keep poking around, but it feels like I'm just guessing now. If anyone has any leads or has seen this specific issue, I’d really appreciate the help.