
Basically to boil it down, the RD45 can replace the RD4. If the screen you have is of an old software version then you will need to get a newer one that can show USB Bluetooth and hands free symbols etc. (SW version 71 or newer IIRC)
I bought the RD45 and newer screen on ebay.
I also had to install a microphone and run my own wiring as I couldn't find the other end of the existing microphone wire behind the dashboard.