You will be able to recode an A4 to an A3. Before buying I would look on
www.navplus.us as there is one member who has fitted an RNS-E to a
Leon Mk1...
I would really look in to this before buying.
On another note, just to encourage you to do your research I just enabled the Speech Dialogue System on mine which now lets me control the nav, radio, phone and cd via voice. e.g. "Specify Town... M-A-R-M-I-T-E-H-A-M-P-T-O-N.... Specify Street.... A-R-M-P-I-T-T R-O-A-D... Start Route Guidance... Show Map... Zoom In..." or "Play station Radio 1". This is a very easy hack to do and works really well.
When the Exeo comes along all of this will be very relevant...