Hi guiv,The major changes are exterior look, with new panels everywhere except the roof. The GU 4 is more squareish looking (which is good) with color coded flairing and bumper (which is bad).
Interior also changes with grey interior, pretty good in my opinion.
Engine wise, the TD42Ti is same carried over. TB48 has been detuned a bit (few kw) I think is to meet emission law or something. ZD30 has improved lubrication and a different turbo (still Garratt VNT), manual one produced 380NM (on paper) compared to 354NM in series 3 and auto)
For ZD30 manual, diff ratio changed from 4.375 (series 3) to 4.11 (series 4). Auto remained the same 4.375.
There are other minor changes like series 4 ZD30 does not have vaccum canister....
sorry to hijack granto
you mention the roof did not change does this mean gutter mounted roof racks from a series 3 will fit a series 4?