Engine misses on #4 (closest to firewall)

Mazda B2600 pickup
220,000 miles
5 speed
2 wheel drive

The engine has always purred, started immediately, ran fine.
Developed a intermittant miss-fire that became a full time miss over a couple of days.
There is gas and spark.
There is no coolant leak - head was checked for cracks, gasket replaced, valves seated, cam OK.
Never overheated, frozen, so block crack not suspected, no sign of cross-over between coolant and oil.
Grey smoke pours out exhaust pipe while running on 3 cylinders when warm.
Replaced # 4 fuel injector.
Noid in #4 fuel injector cable glows continuously, does not flash, as though continuous flow.
Is it possilble that ECU is sending continuous signal to injector, thereby flooding cylinder?
Would this account for grey smoke when warm (unburnt fuel)?
Lost?