2000 Mercury Mystique 2.5L Duratec V-6, CD4E Automatic.

PROBLEM - intermittent misfire of cylinder #4 (according to codes read at
local shop).

Seems to cold start with no hint of the roughness, but when driving after
only a minute or two, it will begin missing at the same engine RPM most of
the time.  This has been going on for months, some days much worse than
others.  I did replace #4,5,6 sparkplugs, but that did not change anything.

The local shop guy said it would require plugs, wires, coil and some other
thing, to the tune of $450.00

Local Ford dealer says emissions warranty would not apply (car has 48,000
miles) unless the computer had to be completely replaced.

Another friend said to replace the wires and see if that works, but would a
bad wire produce the problem I have described?

Any guidance?

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Thanks