As you have said, it is probably cheap motoring, pay £5000 for it, drive it around until Easter 2014 and sell for £4000 - simples. I dont think buying it carries that much risk, so long as you have fully HPI'd it (I would do this myself and not leave to the dealer) and I would also call the Audi dealer where it had it services (at least the last one) to double check the history and what they did. When it comes to sell make sure it has one years MOT and tax to last until at least the end of summer 2014 and someone will snap it up for summer next year.
EDIT - is it full AUDI service history?
Just done a CAP valuation:
A4 CABRIOLET 3.0 Quattro Sport 2dr (ID:25616) YF04TSZ
Date of Registration:27/07/2004
Trade In Clean£3,775 -
Trade In Average£3,425 -
Trade In Below£3,075 -
and we buy any car say, £3280, which is rock bottom book price - i suspect because it is a uneconomical 3.0 petrol.
So, just buy it!