I think, the Audi is rather a brand new A8. For an A4 it's too big.

Do you can identify diplomatic cars by their number-codes on the plates, from which country they are?

With German diplomatic plates it's possible. There it's the number in the middle (code 0 - xxx - xxx).
I have a 0-111-27 in my collection. So this was the 27th car from the Polish embassy.