Hi,
firstly I dont think leaving the key in the ignition is a good idea anyway with modern cars, you never know if they go properly "to sleep" of if somethings stay active.
anyway, I´ve found that hooking the C4 up to a "smart" charger, eg CTEK, prevents any nasty surprises with cars which aint driven very often. my convertible often sits for some weeks till I ve got time AND the weather is fine... the battery is kind of small for such a big engine anyway, my 3ltr Merc Coupe got a similar size. then, modern cars (by which I mean cars that simply go "off" when turning the key) do have sufficient residual power requirements so that the battery is unlikely to be weak after 3-4 weeks. In the C4, the engine ECU will remain on (but asleep, hopefully) as the ECU memory needs to be powered, the VATS anti theft system needs to monitor if the doors are opened without the key (I did once....and the car panicked as required and I had to rush indoors to get the key...)
battery in my 1992 Merc is weak after 4 weeks, in my (modern) Harley after 3 weeks. so... get a charger.
cheers
Wolfram