1998 Saturn SL2 98 Saturn SL2 idles fast in gear

My 1998 Saturn SL2 idles fast in gear when it warms up. The temp guage never goes above 1/4. I've replaced the ECT sensor with a part from Autozone. I've done that twice. Local mechanic said only the factory ECTS should be used. Local mechanic replaced IAC valve and cleaned intake throttle body and flushed injectors. I still have the same problem. Also have a misfire in cylinder 4 (P304 code) Have replaced spark wires and plugs. Still have miss. When warmed up the idle goes as high as 3,000 when I change from D to N. Then it slows down to a rough 700-800 RPM in neutral. As soon as I put it in gear the engine races and I have to hold hard on the brakes to keep the car from moving. If I put it back into Neutral, the RPMs come down after a few seconds.

I'm stuck. Where do I go from here? I can't afford a trip to the dealer. I already "wasted" $600 to have the IAC replaced and it didn't fix anything.