A4 1.8T 2WD to 4WD

it would be way easier, and save you a lot of time, money, energy, and potential problems in the future to just sell your current a4 and buy one with quattro from the factory. most of the mods you have can probably be put onto the new car.

dont waste your time doing it, just get one that already has quattro.
 
It would not just be the rear subframe its suspension, gearbox front subframe suspension its all different between the two cars, if you can get a smashed quattor cheap then maybe its worth it other than that i would not bother tbh.
 
On a B6 / B7 car, the floorpans are the same, the 4WD rear subframe bolts in place of the 2WD item.

On a B5 car, not worth the grief.