If the indicator lamp 
flashes while the Audi A4 is in motion, the ESC or traction control system (ASR) is intervening.
flashes while the Audi A4 is in motion, the ESC or traction control system (ASR) is intervening.If the indicator lamp lights up, the ESC system has been switched off for system reasons. In this case, the ESC can be reactivated by switching the ignition off and then on again. If the indicator lamp goes out, this means the system is fully functional.
lights up, the ESC system has been switched off for system reasons. In this case, the ESC can be reactivated by switching the ignition off and then on again. If the indicator lamp goes out, this means the system is fully functional.If the indicator lamp 
lights up, the ESC system has been partially or completely switched off via the 
OFF button Link.
lights up, the ESC system has been partially or completely switched off via the OFF button Link.Stabilisation control (ESC/ABS): fault! See owner's manual
If the indicator lamp and the ABS indicator lamp 
light up and the message appears, there is a malfunction in the anti-lock brake system or electronic differential lock. This will also cause an ESC malfunction. The Audi A4 can still be braked in the normal way (however the ABS control function will be out of action).
and the ABS indicator lamp light up and the message appears, there is a malfunction in the anti-lock brake system or electronic differential lock. This will also cause an ESC malfunction. The Audi A4 can still be braked in the normal way (however the ABS control function will be out of action).Drive to a qualified workshop without delay and have the fault rectified.
WARNING!
If the brake warning lamp lights up together with the ABS indicator lamp and the ESC indicator lamp, this can mean that the control function of the ESC/ABS is not working. Input from the functions that stabilise the Audi A4 is no longer available. This could cause the Audi A4 to skid sideways. Drive carefully to the nearest qualified workshop and have the fault rectified.
Note
For more information about the ESC and ABS systems, refer to Link.