I just got back yesterday....Loved Rothenberg but would have to agree with Michael...4 days seems like a bit much. I think you could stay just 2 nights and see something else. Rothenberg is beautiful..quaint...very historical... lots and lots of shopping but you could easily see it all and not feel rushed with a 2 night stay.
Make sure you "walk the walls"....and go to the Kathy Wolfhart shops...the Christmas ornaments and Pyramids are wonderful. You could skip the "dough balls" though...they were terrible! They look great...you see EVERYONE eating them...but YUCK! You could probably spend one of the extra days in Trier...if you like history. If you like to do flea markets....there is a great one in Tongren, Belgium (not far from Maasrtich. It's on Sunday... great antique shopping. There are not a lot of accomodations..so plan ahead if you choose to go. I stayed at the Albi...it was very nice.
One of the Rhine River tours would also be worth doing...Or go to Heidelberg and climb the hill to the castle. It's very beautiful.
Since you are driving....be aware that there is A LOT of road construction in Germany...which narrow the AB down to one or two very narrow lanes. Take care in driving...keep to the right...because I guarantee...there WILL be someone driving faster then you.
Enjoy you trip and your new car!

we just stayed in bruges for 2 days. perfect. stay at the duc d'bourgogne. beautiful canal view in room number 4. hotel room and bathroom huge for the price. eat dinner there too. a little expensive, but wonderful food and great canal views. romantic.