An mechanic told the the following:

The Federal Government considers VAG, i.e., VW and Audi, as one manufacturer. The Federal Government allocates a certain number of diesel permits per manufacturer; thus, VAG gets the same number of diesel permits as MB and BMW despite the fact that they have two large makes.

Thus, VAG has determined that more diesels should go to VW rather than Audi.

So, you can kiss your A4 diesel dreams good bye. VW as "the People's Car" gets the diesels. So, if you want a VAG diesel get an SUV or a Jetta/A3.