How to convert new 4x4 crawler to tracks?
I started with something based around the tracks that came with the original LEGO Mindstorm's kit: 680 Technic Tread:



I then build a rather more bulky than I'd have liked set of beams around it - I think if I'd have really dug out my Technic elements I'd have had more success making it look closer to the mattracks options:



Here's the very bulky rear - this would obviously want to be pared down somewhat, and you might want to either power the top wheel, or both the bottom ones:


Here's the rear with the beams removed:


You would need to have the driving axle on a universal joint to allow cornering, and would want to mount the whole thing on the mounts for steering. 

I think it would be better if I'd had more of the newer Technic beams (the ones without the studs - I could have then connected them with two coat-hanger pieces and have another one vertically to the top.