Don't know on the 4 series tractors. But I have a 3 series 4WS and I remember there was quite a bit of difference between it and a 2WS, even the wheels and tires were different sizes between the two. Sounds like a big project with a lot of proprietary parts. Maybe try to find a "donor" 4WS if you want to do it, or trade for one if you just need 4WS... sounds costly either way.