I had an A4 in my 30' Morgan 25 years ago, but I will throw out some ideas.
I would put in new plugs, with correct gap.
Check that the observed spark on each plug is blue-white and not yellow.
Run the engine 5-10 hours and check the new plugs.
While running and fully warm, measure the engine temperature with a IR thermometer. I presume that you have the raw water cooled model- the temp should be about 165-170 measured at the thermostat housing.
If the ignition wires, spark quality, and temperature are all good, but you still have #1 & 2 fouling with 3 & 4 clean, then I would check one last thing- This is a long shot.....
The cooling water on this engine splits into two paths. One path goes to the thermostat housing bypass and the other path goes to the plate on the side of the engine to cool the block & cylinders. When the engine is cold, the closed thermostat blocks the water from the cylinder block (except for a trickle) but allows full flow of water to the exhaust manifold through the thermostat bypass and then out of the engine.
As the engine comes up to the thermostat temperature, the T-stat opens and as it opens the water flow from the cylinder block increases and the flow through the thermostat bypass decreases, but all water still goes to the exhaust manifold.
Now when the water enters the block through the side plate, cylinders 1 & 2 are nearest the thermostat where the water exits from the block. So the water will tend to cool cylinders 1 & 2 more than 3 & 4. Although it doesn't show on the drawings of the engine or in the parts list, my engine had an elbow on the inside of the water inlet plate that directed the water toward the bottom of cylinder 4. This was done to cool all of the cylinders more evenly.
So you could check to see if you have such an elbow and that it is pointed in the correct direction.
If you don't have the internal elbow or if it is not pointed toward the bottom of #4, that could cause cylinders 1 & 2 to run cold and 3 & 4 to run hotter. That could explain the difference in the sooting of the plugs.
I realize that this explanation is complicated, but if you reread it a couple of times and look at your engine documentation, I think it will make sense.
Good luck.
