Keep the S4.
In a trade in, you will get shafted, the trade in value will be many thousands less than you could sell it for privately.
As far as your daughter ( I assume recently new driver, 16-20 ? ) NO... An S4 is not a good idea, especially when you put her friends in the mix. The temptation to have fun is too great.
Had my Dad let me have the 600hp 67 Camaro when I was 18, I would probably not be here today. Or I am sure I would have had that conversation that starts " Uh Dad, had a little mishap with the car today .... Gulp !! "
The Jetta should be the option as the daily driver and all around family transportation. Sounds mundane but it is the wiser solution. The S4 is really a 2nd car, after 3 years with mine, I will not have an S4 as the primary Audi again once I find that 2.0 or 3.2 Avant.
As far as the commute, I have a similar commute and it is a lot of cash thrown into the tank every week, no to mention the maintenance servicing costs increase dramatically.
Love driving the S4 on long trips, not so much as a daily driver.