It's not a simple job integrating the controls on the steering wheel into a
different radio/CD head unit. The radio is connected to the vehicle's CAN
bus (Controller Area Network
http://en.wikipedia.org/wiki/Controller_Area_Network) and receives data from
various parts of the car. The buttons on the steering wheel will be
transmitted over the data bus and the radio picks these up and acts
accordingly. The BSI will also transmit control packets over the data bus so
the radio can adjust the volume as you drive the vehicle, or mute the
radio/CD volume if the parking sensors are operating or Bluetooth handsfree
kit is fitted and an incoming call is received.

You have to remember that all the buttons on the steering wheel are all
software programmable, I would be very easy for Citroen to recode the radio
buttons to wind the windows up and down. If you have the non base model of
the C4 you'll have the user programmable button on the steering wheel (hold
it down for 2 seconds to program). You can use the button to switch the
interior lights on/off, display the event log or dim the console
lighting....

I don't know the exact details about the radio in the C4, but I would guess
that the BSI interrogates and reads the current status of the radio so it
can drive the centre console display allowing you to see details about the
current radio station or the CD track been played.

I've just had a quick Google and I've found referenced to
http://www.bluespot.co.uk/install/show_model.asp?id=42 and this mentions VAN
bus, I'm pretty sure that the C4 has only CAN networks (anyone confirm?) so
I've not found any adapters that I think would work.

So, yeah, all in all - not a simple job, but I'm sure someone will develop
an appropriate interface eventually.

Chris