The A4 was redesigned for 2017 – and the only major change to the back seat is that now the buckle for 2C is flexible and is on a short seat belt webbing stalk – which makes it taller than the buckles for 2D & 2P which are rigid and flush with the seat cushion. This will slightly improve the chances of getting a car seat installed in 2C. The center remains exceptionally narrow – which will continue to limit what car seats can be installed there… making most still too wide to fit in 2C. 
The 2009-2011 back seat looks just like the 2012-2016 except for the 2012-2016 head restraints don’t protrude forward nearly as much. Otherwise the back seat is identical to the prior model year range.
The A4 was redesigned for 2009, and the center became unusable for most car seats, sadly – as Audi made it narrower, and made the center buckle directly in front of the driver’s buckle (and all the buckles are rigid and flush with the vehicle’s seat cushion).
The 2009-2011 back seat looks just like the 2012-2016 shown above, except for the 2009-2011 head restraints protrude forward.
The head restraints were changed for 2006 – now they protrude a lot more. Otherwise the back seat is the same as the 2003-2005 A4 sedan.
Lower anchors were first introduced in 2003 – and are for 2D and 2P. Otherwise the back seat is the same as the 2002.
The back seat is identical to the 2002… except importantly there are now lower anchors for 2D & 2P.
The A4 was redesigned for 2002 and has two important new safety features in 2C – there is now a shoulder belt (before there was only a lap belt) and there is now a head restraint. While there are tether anchors for all 3 seats, there are still no lower anchors for 2002… those will come for 2003.
The back seat is identical to the 1996-1999 – with the only difference that now there are tether anchors (that come factory installed) in all 3 seating positions. There are still no lower anchors – those won’t come until the 2003 model year.
The lap belt in 2C has an automatic locking retractor and can be used for car seats – simply pull the belt all the way out, keep it as long as possible as you route the belt through the proper path in your child’s car seat, buckle the seat belt, and then feed all the excess belt (while pushing down really hard on the child’s car seat) into the hole where the lap belt emerges from.
There are no lower anchors or tether anchors in this vehicle. However, tether anchors can be retrofitted for all 3 seating positions in back – and we STRONGLY recommend getting the tether anchor retrofitted if you will have a forward-facing child riding in the back seat (rear-facing kids and those in boosters who are using the vehicle’s seat belt across them as their restraint do not need tether anchors).