P0304 Misfire

Hello guys. My 2004 Ford Fiesta 1.3, engine code A9JB, is experiencing a misfire on cylinder 4. I've tried everything to solve it but nothing worked. The misfire is intermittent and it comes sometimes especially at low revs (1000-2000 RPM). Mechanics told me to replace spark plugs, ignition coil and wires. On ignition everything is new. Before replacing wires I've tried moving spark plugs and wires from cylinder 3 to 4, but nothing has changed. After that I tried to make a compression test at a local service. They made a bad one, but at least all the cylinders had the same pressure. The last thing that I had left was the fuel injector. I took it out, cleaned it and put it back (my bad was that I didn't move it to another cylinder to see if the code moves). The thing that I'm thinking now is to take out the injectors again and move the injector from that cylinder to another. What else could I check? The fuel trims look good on the diagnostic tool so that's why in the first place I didn't think about a fuel problem. Also, the injector resistance is equal on all 4 injectors. The spark plug on cylinder 4 looks like the A/F ratio is a little too poor. In the second photo you can see the spark plug from cylinder 3 which is running normal, even if it has a little burn mark.