you can buy 'wobbly bolts' which cater for this. I'm new to Audi forums as picking up an A5 later in week but still have my Seat Cupra K1 and have seen this discussed on the seatcupra.net forums. If I can find the thread and a link to the site I'll post it up. Maybe google will also find something.
You will need to know the PCD of the A4 and A5 to make sure you get the right ones
Aswell as PCD you also need to make sure the spigot size is the same (all covered in the thread(s) above.
If the offset in PCDs is too much you will need to buy 'PCD Adapters' but they will push the wheels out further and with 19" rims this may push them beyond the wheel arches or cause them to 'foul'
What are the PCD's on A4/A5's anyway out of curiousity?