It was exactly the same on my 307 (which I am certain has identical engine to your C4). I even had the fuel tested when the De-pollution warning came up on mine. It was according to diesel specs and not the cause of the problem. Your problem is highly unlikely associated with using bio-D.

In my experience, Peugeot/Citroen dealers are not very competent at diagnosing faults at all. They simply do not know how to interpret the "real" cause of the fault codes they read from the analyser.

On my first return to dealer after the warning came up, they simply reset the computer and said "try it again". Within 2 days it came up again so I took it back. Reset the computer again and within a week back came the fault warning. This time I did my own research and after some questioning of other owners who had similar problems I came to the conclusion that the exhaust pressure sensor was the most likely cause. I insisted they change it and they did. Result - no more problems.

Here is the thread for the most convincing saga which convinced me http://www.aussiefrogs.com/forum/showthread.php?t=42958