My 1991 Audi 80 will not start, engine cranks

What is missing from the firing;
Air, Fuel, Compression and spark.

Depending on the year make and model, a DTC (diagnostic trouble code) would help. Is the check engine light on when it does run? Testing needs to be done. Check for spark at the plug, make sure fuel pressure is what it should be and that the fuel injectors are working. If all that is good, I would check compression and timing.

check the ignition system, when was it last serviced?
could be it needs new plugs or leads? Fuel pump changed but is the fuel system in good order. A GOOD local mechanic should be able to advise you on the issue

Changing the plugs probably had nothing to do with it.
White smoke is usually a sign the engine is burning coolant or trans fluid. You might want to check those fluid levels.
If the coolant is low, you may have a leaking head gasket or intake gasket.

have diagnostic codes checked to see if you have a cam or crank sensor fault. make sure fuel pressure is at the required pressure, just testing to see if you have fuel at the valve is not enough. replace all spark plug wires. if you replaced two then all are just as old and even when they "look good" does not qualify as "they are good". Replace coil pack. start up drive away happy

did you reset the alarm/immobiliser system every time the power was disconnected???

Pull one or two sparkplugs and check them for moisture; not fuel, but for water beads.
I don't know how long your engine ran after losing coolant but there is a chance that you lost the head gasket and have gotten water into the cylinders.

check crank position sensor, run scanner for trouble codes , some auto parts will do this for free

The cylinders are numbered as on the spark pack you changed. You might have done better putting the seafoam in the tank as an injector cleaner. Not sure why you changed so many parts, you seem to know how to get the codes from test connector lead hookup and the blink pattern. You might also want to check the fuel rail pressure and the crank and cam angle sensors. Before replacing the injector, swap the #4 injector with the adjacent one and see it the code moves with the injector or stays put. If the code stays on #4, then check the injector wiring, if it moves clean or replace the injector.

Sound like a bad ground wire going from engine to firewall...