Ford Focus Engine light troubles?

I own a 2000 Ford Focus, and the engine light stays on, despite being maitenced for the cause 2 different times. I've spent nearly 700 trying to fix whatever issue is, I need this check engine light problem to be resolved due to smog. My mechanic thought the sensors were bad, so he replaced the oxygen sensor, the motor mounts, then transmission mounts, then spark plugs, then wires. After a while, he just told me there was nothing else he could do as the codes weren't adding up (cylinder misfire 4)

Right now, I really don't know what to do, any advice is appreciated thanks.

Your mechanic is an idiot first off, motor mounts? Nothing he can do? For starters, swap injectors and run a compression test. Gotta verify the cylinder is capable of firing before you diagnose a misfire

Find a new mechanic that can test the ignition coil, fuel injector and cylinder compression

Replace the ignition coil pack, (about $22 from amazon) if sparkplugs and ignition wires have been replaced or the wires ohm out withing specs. The stock coil packs crack and start causing problems somewhere in the 120k mile range. Cheap and easy fix anyway. Ditch the mechanic he is an idiot - or take it to a dealer and get it diagnosed and repaired correctly the fist time.