Hi Stephanie,

You've not told us model years or engine sizes, etc, so it's impossible to answer this accurately. 

However, the Audis A4 and A6 share a lot of components, so it is possible that, in certain cases, their gearboxes are interchangeable. I'd be very slow to do that, however, as there may be subtle differences between them, even if they physically fit. Not only that, but the control units for the automatic transmissions will almost certainly be different, and they often interface with other aspects of the car.

In summary, it might fit, but we'd not recommend this.