I am a Volkswagen Technician and will answer you Honestly. Please do NOT service the fluid in your 4 speed AG4 transmission. That specific model of transmission has been a rotten egg since its induction in 1993/4. If it is not leaking (low fluid) then I would NOT replace the fluid nor the filter. IF you do, you will be removing all of the clutch pack material which is floating around in the fluid (this is allowing soft smooth shifts) and once that material is gone, the distance bewteen the clutch disc's and metal disc's (inside) will be greater and now the shifts will be noticably harder. Then the next step is internal failure of more clutch disc material and ultimately complete failure. I have seen this same thing happen more times then I care to admit. There are ALOT of dishonest people who will tell you to change your fluid just to make a buck off of you or because they "hear thats what your supposed to do". Did you know that in your Owner's Manual states that the Automatic Transmission fluid is LIFETIME fluid? So why would they recommend it unless there was a leak?
Anyhow, to make it short.... Do NOT just change the fluid unless there is a problem or leak.
10 Years Volkswagen and ASE Master Technician