Ford Repair: 2000 Ford Explorer, piston rings, coil pack

Question
I own a 2000 Ford Explorer with 130000.  For the past 6 years or so, it has started missing around every 10000 miles.  I usually take it somewhere and always get the same message, "cylinder 4 misfire".  I have had several tests and things repaired and done.  Replacing a single spark plug always fixes the problem.  I have been told that the problem is more than likely in the engine.  I have opted not to get the engine looked at because I've heard that that is expensive.  And at this point I am content to pay 60$ for a new plug once or twice a year.  

The above is not my question, just thought it could help in the diagnosis of today's problem.  

This morning in the middle of a turn my car just straight up quit.  I pulled to the side of the road and tried to crank it back up.  Cranked great and ran fine until I got on the highway.  Once I got up to about 65 mph it started shaking violently.  I know painfully little about cars, but the shaking seemed different from the missing that I am accustomed to from the afore mentioned problem.  Once I got off of the highway and to a lower speed, it ran fine again.  Same story on the way home, fine at lower speed and extremely shaky at higher speed.  

Any ideas about what could be wrong?  Again, I know next to nothing about cars.  Thanks in advance for your help.

Answer
hello there. i have a stong feeling that both your problems are connected. This cylinder 4 misfire you have going on i bleieve is a real bad concern!!!!  when a plug is being wiped out that often, there is probably bad piston rings in that cylinder letting engine oil into that cylinder and fouling that plug. mow if oil is getting in that cylinder that meen fuel is also getting into your engine oil. IT IS NOT a good thing to get anything in your engine oil as it will cause more mechanical problems then you want,
when a cylinder starts to misfire because of a fouled plug. it puts alot of strain on the circuit for the coil pack that provides the spark for that cylinder. If you put strain in that circuit, then you are putting strain on the driver circuits for the coil packs in the power train contol module. this module is worth 1250 -1750 dollars!!!! this is not something you want to replace because of neglect!!! I would really have a compression test done (wet and dry) and a cylinder leak down test. this will show you the condition of the engine in the mechanical aspect of things. Once mechanical aspects check out ok then you can determin if there is an electical fault. let me know what happens. good luck
Chris P