The service engine soon lis on and the wires/plugs have been replaced twice within the past 8 months because that was the diagnosis. The light is still on and I had them test it today/they say it is the 4th plug again. It can't be not already. Then he said it might be the ignition coil. What do you suggest? It has 66,000 miles and today it jerked a little.

Was it diagnosed as the 4th plug? Or the 4th cylinder? My friend had a '99 plymouth breeze with around 70k miles on it, with a hole in one of the pistons, yes a HOLE in the piston, and that would certainly cause a major compression problem, if that's what it's diagnosed as.

He said it was the 4th cylinder.

Well if you pull out the plug, and it's fine, then it's probably an internal problem. You should take off the head and take a look inside.