What i found out when off roading my 04 D3 and a friends 12 D4, the terrain response system has been changed.
In rock crawl, my centre diff is either open or closed. No inbetween.
The D4 has it in stages, 1/3rd, 2/3rds and fully on. The same with the locking rear diff.
I think the later D3's have this update but i'm not 100% on that. The D3 made it over every obstacle we met, but i think it was made a touch easier in the D4.
The D4 doesn't have the grab handles built into the front seats to help rear passengers climb in, something many people with kids miss.