The actual lens geometry is different. The above link goes into it in great detail.
The lens is designed with the shape of the burner in mind, and the burners are completely different shape.
The plastic housing will be identical sure, but the actual projector lens is not the same. I'm sure if you pull your spare RS4 housing apart an analyse the lens you'll find that out. Bear in mind the geometry of these lenses is so precise, that even the Depo aftermarket units, which are insurance approved, will fail some of the more stringient beam pattern tests. Its not something your going to be able to notice with a bit of a squint thru the hole in the back of the light unit.
JCB: if you had actually read the link i posted, the guy actually mentions the beam cutoff line in projectors like ours giving the appearance of a "perfect" beam, when infact its not the case.
If you had RS4 HID's then you should have picked up an aftermarket ballast and installed them using the correct parts.
Your both conveniently ignoring the fact that the entire reason load levelling and washers are required by law is because HID's are glarey, and you dont have them fitted, meaning that not only is your car most likely producing more glare than a halogen car, but also more than an OE HID'd car, due to the missing components and the wrong lens fitted.
Its also perfectly fine to use them in reflector headlamps, as long as theyre designed for it. Vectra B's and facelift Corsa-C's have a HID option from the factory, both utilising normal open reflector headlamps.