open the trunk, on the side by the opening you will see a round cover, pull that cover back, that will give you access to the screw that removes the tail light assembly. remove the tail light assembly, when you pull out the tail light assembly you will have access to all of the bulbs in the tail light. this is a 5 minute job, very easy.
Replace the brake pads.
Pull the carpeting behind the brake light. Unclip the assembly, replace bulb.
This site has step by step instructions (with photos), perhaps the A4 is similar. http://audidiy.blogspot.com/2010/04/how-to-replace-brake-light-bulb-on-audi.html
As of 2014, the cost for a Audi A4 brake light is between 12 and 15 dollars. This light can be purchased at stores like Auto Zone.
The brake light in the Audi A4 display means there is an issue with the brake system. The most common cause of this warning light is low brake fluid.
Audi brake switch same as VW Golf (in newer models). A common problem with VW Golf 2002 is the brake light switch. The vehicle won't come out of park if it breaks. The 2002 model had a recall due to faulty brake switches. Read online how to fit it before fitting - easy to break if you don't understand what you're doing.
You need to replace the break pads or you can bodge it up by connecting the two wires
you go.... and replace it!
By going to the nearest service!
Faulty ABS unit .
in my 2001 A4, it was the hazard light switch that had to be replaced. maybe thaqt's your issue also
replace brake line