To the best of my knowledge, this page accurately describes the procedure that I followed to check the electrolyte level on the stock battery in my 2000 A4 1.8TQ. I cannot confirm that this procedure will work for your battery.
Use these instructions at your own risk.

• Flathead screwdriver for battery caps.
• Distilled water.
• Flashlight.

Audi A4s do not ship with a sealed maintenance-free battery; therefore, occasional maintenance is necessary. There's not much to do except check the electrolyte level a couple times a year.
 

1. Remove the battery caps.

Remove the plastic trim between the firewall and windshield to get access to the battery. 6 battery caps (3 on and 3 off in this photo) Using a flathead screwdriver, remove the 6 battery caps.

2. Check water level.

Check the water level in each of the receptacles (you'll need a flashlight to see). Top off each one with distilled water up to the maximum water level mark. Do not use tap water and do not overfill! Replace the battery caps when finished.