RPMs stuck at 4000 with check engine light

[DWosik13]

My car has been seeming to be running fine lately but i was late for work one morning and decided to go a lil over the speed limit once i hit 80 my rpms just stop at 4000 and the car loses any chance of going faster. the check engine light usually comes on than off but i got it to stay on one day took it to autozone they scanned it and said it was a cylinder 4 misfire so i went home replaced the spark plug on 4 and than went out to test it and yet again same problem any ideas?

 

[Sabrewings]

Did you change the plug wire?

 

[Meemperor]

Plug wire, coil or bad cat.

 

[Reptile]

Ignition module....if a coil was bad, it would be on two cylinders.