If anyone interested (and I am amazed I have not seen this come up before), I learnt a lot today from an uncharacteristically helpful Audi Technical Support gent.
To recap, I have a 23 plate Q4 e-tron S-Line. The manual and the Audi app suggest the vehicle has Adaptive Cruise Assist - the car even has the steering wheel stalk with the distance setting rocker switch.
The problem is the ACA doesn't work. I have normal cruise control but not the bit where it slows down automatically and then speeds up again.
As mentioned here, and by Audi themselves it should be a Function on Demand available to purchase except when I access my app, all I can buy are Themes (incidentally these do not work either - that is another matter!).
So what's the problem? Well it seems Audi have massive issues with the app and the country in which it is being used. I obviously logged in and set UK as my country but the Audi Tech guy told me to log out, and log in with Germany as the country. The app then loads and shows everything you'd expect except displays German language. I managed to navigate to the Functions on Demand shop and sure enough, available to purchase for my car was Adaptive Cruise Assist. I didn't buy it as it was priced in EUR and given my experience trying to buy a Theme that didn't work, I have refrained until they fix this problem.
