I'll post it in english for the non french-speaking readers, though I'm a frenchman myself.
Here is a list of french cars supposed to be in GT4 :
Citroen C5
Citroen C-Airdream 02
Citroen Saxo 1.6I VTS
Citroen Saxo Rally Car
Citroen Xantia 3.0i V6 Exclusive
Citroen Xantia Rally Car
Citroen Xsara 1.8I 16V Exclusive
Citroen Xsara T4 WRC Rally Car 03
Citroen ZX Raid Car Paris Dakar
Peugeot 106 1.6 S16
Peugeot 106 Rally Car 94
Peugeot 205 T16 Rally Car 85
Peugeot 206 GTi 00
Peugeot 206 Rally Car
Peugeot 405 T16 Vatanen Pikes Peak Car
Peugeot 406 3.0 V6 Coupe
Peugeot 406 Super Touring Race Car
Peugeot 607 Feline Concept 00
Peugeot 905 LM Race Car 90
Peugeot Oxia Concept
Renault 5 Turbo 2 84
Renault 5 Turbo Rally Car
Renault Clio Cup Race Car
Renault Clio Williams
Renault Espace F1 Concept 95
Renault Laguna Touring Race Car 99
Renault Megane Rally car
Renault Clio Sport V6 Evo II 03
This list is extracted from an exhaustive list that has been posted on a german forum (granturismo.de) and it looks realistic despite the fact that it doesn't feature the Berlinette (and we know from the pictures it'll probably be included).
I also read about a Citroen CX in the game.
