The B8 S4, 2010 to 2016, has the 7-speed S tronic, the DL501. Inside it is the mechatronic. That is the transmission computer, the solenoids and the sensors in one unit, and it sits in the transmission fluid.
On the earlier cars the sensor plate inside the mechatronic is a known problem. The circuit cracks from heat and vibration, or the speed and position sensors start to read wrong. First the fault comes and goes. Later it stays.
The signs are a blinking gear display, hard or missed shifts, often in second, third or reverse, or no reverse at all. It can go in limp mode and drive only on some gears. You clear the code, everything looks fine from outside, and the fault comes back.
The repair is inside the transmission. This is not a sensor you change from outside. The gearbox has to come out of the car and the case gets split. A new mechatronic from Audi costs a lot. Many times the better way is to repair the mechatronic. I change only the parts that failed, and after the repair I adapt it to the car.
There is no maintenance that stops this failure. But change the S tronic fluid and filter every 40,000 miles, 60,000 at most, and do not wait when it starts to shift strange. If you keep driving with a bad sensor, the clutches and gears wear too.
If your S4 blinks the gear display or loses reverse, bring it. I read the full transmission data and find what failed before anything comes apart.