Just looked at Erwin running gear manual. On the Front wheel drive A4's the disk and rear hub are one component and thus you cannot separate the disk from the hub. This is not the case on the 4WD A4s - you can replace the disk without removing the rear hub.
Unfortunately, on the front wheel drive if replacing the disk strictly speaking you should fit new bearings into the new hub even though your old bearings could be ok. The reason for this is that generally the bearings are hydraulically pressed into the hub. Or if you're me, carefully hammered in using a large socket or something of similar diameter to the outer race of the bearing so that you do not place any stress on the bearing while driving it in. Therefore, to remove the old bearings takes considerable effort if you want to save them. This usually involves carefully hammering out the outer race of the bearing by means of a large blunt screwdriver. There is usually a small slot machined out off the bearing stop in the hub, which will allow you to place the screwdriver onto the outer race to start the driving out operation. The slot could be difficult to see for all the grease.
Having said all this. The bearings are usually only about £10-£20 and quite frankly trying to save the old bearings is an act of desperation.
Also, if you don't have a hydraulic press and you don't fancy hammering them, most garages should have a general hydraulic press or even a large vice and can perform this for you in minutes.
I sometimes warm the disk up in the oven to at least 100degC. in an attempt to expand the hub to make the driving operation easier. Make sure you clean all the oil of the new disk or you'll stink the house out. NOT popular with the rest of the household!
As for the ABS sensor ring, this is also pressed onto the hub. From past experience I would nearly always recommend replacing this at the same time as it's such a lightweight piece of metal that it's only relay good for one installation after that it's stretched and just won't remain tight on the hub. Once loose this will screw up the ABS sensor and report it's self as a fault. Hope this is of some help. I strongly recommend getting a Haynes manual if you've never tackled this before or the Audi Ewin manuals if it's a Quattro since Haynes don't cover the Quattro /ubbthreads/images/graemlins/dancing.gif