OK so it's a 99 4.0L with 115K on it & the Tech has replaced the plugs a number of times & cleared the misfire codes & it'll run ok for a while, then the rough running returns in about 5-6 months with cyl misfire codes again.

It's running poorly again & the same cyl has fouled it's plug twice & replacing that plug doesn't clear the problem & it's running poorly at 35-40 mph & coughing & chugging so bad that it not driveable.

He may have more than one problem & I agree his Tech isn't looking deep enough to find the root cause problem.

So list ALL of the trouble code Numbers he's had.

With 115K miles on it, a number of scheduled maintenance items are due, so where is he on ALL past & present due scheduled maintenance items???

Good answers to All of the guys previously asked questions would likely help narrow down the guess list.

Right now it would be helpful to have a really good "plug read", to know what kind of plug fouling he's had & especially on the cyl that's now fouled it's plug twice.
Again, how are the plugs fouled, carbon, oil, ash, cooalnt, ect????

All kinds of possibilities abound, so we need more detail to offer up a better guess.

He could have a mechanical problem(sticking rings, bad valve stem seals, leaking fuel injectors, ect), electrical (bad plugs, bad wires, acting out colpack as you've proposed), wrong part, (wrong spark plugs or wires, wrong oil weight, wrong PCV valve), Bad worn out dirty parts (Lazy O2 sensors, dirty MAF sensor, clogged up PCV valve, dirty or worn out fuel injectors, bad pug wires, bad TPS sensor, clogged up air or fuel filter), or some combination thereof.
So all kinds of possibilities abound, at this point, you really need more detail & imo it seems to me a good "plug read" sure would help narrow down the possibilities at this point.

Most autoparts stores can bench check the coil pack at no charge.

A bunch more thoughts for pondering.