A 365 GTC4 uses one liter. I would think that little changed as far as A/C capacity between the C4 and the 400 / 400i.
It depends which A/C system the car has. The single system (front only) takes 1kg (2.2lbs) R12. The dual system (front and rear) takes 1.5kg (3.3lbs) R12. The refrigerant capacity of the A/C system is listed in both the owners manual and workshop manual is the same section with all of the other capacities (engine oil, trans. oil etc.) Brian Brown Patrick Ottis Co.