Cd of this A4 is supposed to be 0.27 so i think most of the rear aero will be sorted.
i don't think the fins will aid much in their curent form but perhaps fins and spoilers might be more of a future aid to aerodynamics then we think.
the
bmw BMW Vision EfficientDynamics for example has spome strange C pillar configuration.
but more recent just about every car that came out recieved small dams in front of the tires, and on some cars these dams take on alternative forms wich makes me wonder if small devices wich deflect the air in more favorable patterns isn't going to be something we'll see more often in the future.
for example the odd
alfa romeo bat (batman would drive one) achieve very low drag coeficients with the aid of finlets (with a Cd that low they have to be functional)
i think fins at the rear can have two funtions
1: prevent C pillar vortexes of make the wake less wide,
2: perhaps small central fins, or ribs running in the same direction of the airflow could have a beneficial effect (see the front undertray of the merc)
the problem with all these things is that the airflow patterns