There have been a few conversions, but not AWD. The later T4s (for us) were all cursed with the chain-chunking VR6 bolted to the rando-fail 01P slushbox, so they were ripe for a TDI+manual conversion. But when you do run across one, people want stupid crazy money for them. I had a '93, which was the only year we could get a manual and those were all the 2.5L 5 cyl, which while not as powerful as the VR6, are infinitely more durable. Mine was an automatic, though.
Having had every generation of Transporter sold here, I'd say the best all around vehicle is a tie between the late T2 (1975-79) and the T3 (Vanagon, 1980-91). They are roomier inside, better seating position in front, were available here with the actual Westfalia camper package, and from 1986-91, were also available with AWD. Those command a lot of money, too, but TDI conversions are common on those. The T4 is better at higher highway speeds (70+) due to its longer wheelbase and more narrow streamlined body. But for camping, there is no contest, the rear engined Buses are FAR superior.