VWs 4-motion system is not the same as Audi's quattro system. The Audi S3, A3 quattro and TT use a Haldex system (same as VWs 4-Motion). Basically it's 2-wheel drive when on the straight. Any loss of traction and the haldex unit engages the rear wheels. The quattro system on the other hand is permanent four wheel drive. More details here. http://forums.audiworld.com/showthread.php?t=1986343
I've got a 2001 A4 (B5 model) 2.5 TDi avant quattro sport. The AKN V6 engine generates 153bhp. This is more than ample for pulling the RIBcraft 4.8m. It hauls like a locomotive, due to over 300Nm of torque available on tap, most of which can be utilised from a standing-start to 3,500rpm from the 6-speed box. The later A4 2.5 TDi models came with 180bhp and if chipped could be taken to over 220bhp, although I've heard a few horror stories about them running rough.
The VW Passat 4-Motion is a very nice car. Probably closer to an A6 in size. Roomier than the A4 for starters, (especially for rear seat passengers) and with a load bay that would put a transit to shame. Uses PD engine technology (130bhp), for cars up to 2005. Newer models with 2.0 litres gained another 10bhp. Personally I'd steer clear of petrol versions unless you like garage forecourts!