The car shown here is based on an Audi S4 Avant but is sporting some RS4-like modifications.
This mule looks to be riding higher than a standard S4 due to the larger RS wheels from the RS5, with RS logos on the front brakes (bigger than on the S4). The front bumper has also been changed to accommodate larger air intakes. At the back is still the S4 exhaust but under the hood the car is packing something challenging, we just are not sure what that is.
We can speculate, though, and perhaps it will be a a tuned-down version of the 4.0 liter twin-turbo V8 rumored to be in development for the upcoming RS6 and RS7 variants. For those two models it will come with a power rating in the neighborhood of 600 PS (441 kW / 592 bhp). On the RS4, output may get dialed down to around the 450 to 500 PS (493 bhp / 367 kW) mark.