We were told the A5 uses a shortened version of the A4 platform. However, Audi decided the A5 coupe was not enough and released the A5 Sportback, a 5 door version of the 3 door coupe. We were told the A5 Sportback uses a lengthened version of the A5 platform. Whether if that means it uses the same platform underneath the A4 or not, is still unknown.
Then there is pricing. The A5 coupe is more expensive than an equivalent A4, less expensive than the A5 cabrio and more expensive than the A5 Sportback. The Sportback is more expensive than the equivalent A4 sedan. There seems to be a little logic in it: the less practical it is, the more they charge you for it. Except for the A4 Avant, which is more practical than the sedan and more expensive.
This is what I think happened with all this mess in A4, A5, A5 Sportback: Audi had in mind that the A5 Sportback should be the A4 sedan, but they felt it was a little bit too risky, not enough mainstream. Then, they decided to use this crazy 4 door sedan coupe fashion the Mercedes CLS started to fix the idea. There would be a conventional A4, and the brilliant idea was released as the A5 Sportback.
So, the Audi A5 Sportback is the car Audi never dared to call A4 and now they charge you a 2 grand premium for it. No wonder why nobody is buying it.