Ah, but most of the "advanced" features of a D4/RR are down to extra software and electronics, rather than extra hardware. However you are right - a better comparison would be the G-wagen professional to the D4/RRS. Only 50 to 300 kg difference. Both have a similar (steel) body and a Euro 4/5 3L diesel, however the Disco has a lot of extra gadgets for the weight.
Anyway, back on topic... I think it would be a relatively simple and cheap exercise for LR to whack a new body and different wheels on a diesel D4(5) chassis/running gear platform and call it the new defender. Which I suspect will be what happens, unless Tata has their eyes on future military contracts.
Come to think of it, isn't that what happened with the RRC to create the D1 ;)

