Originally posted by Antonio Wright:
A4/Jetta same exact car.
Passat/A6 same exact car.
Not quite.
The Passat and A4 are much closer kin than the Passat and A6. Passat & A4 ride on very similar basic platforms, with the Passat receiving the same 'zero-scrub' four-link front steering/suspension geometry, but carrying a beam axle rear suspension rather than the A4's fully independent.
Passat and A4 share the same north-south engine configuration with FWD/AWD transaxles, and--up until this year or so with the introduction of the 3.0 Litre V6 in the A4--share the same basic engine choices (S4 notwithstanding).
The Jetta is a smaller, more traditional front-drive platform that places the engine east-west under the bonnet and rides (in addition to other choices, one of which is obviously the Audi 1.8T) an uplevel 2.8 Litre Volkswagen-designed VR6 (as opposed to the Passat's Audi-designed 90-degree 5-valver 2.8 Litre V6).
There is little-to-no platform relation between the VW Jetta and the A4.