Yes you are right. All 4 are placed in Paris (France) when it is liberated. This happens once during the game. So if Germany recaptures Paris and the Allies liberate it again, then France does not get any more free infantry.
By the way, France actually gets 12 IPCs worth of units to place on France. So it doesn’t HAVE to be 4 infantry, but I think that is the most popular solution. It could be 4 infantry, 3 artillery, 3 mechanized infantry, 2 tanks or any combination like 2 mechs + 1 artillery, 2 infantry + 1 tank, etc.
I think you could even choose 1 fighter, 1 tac bomber or 1 bomber, however in the case of the fighter and tac you would be losing 2 IPCs or 1 IPC respectively.
Also, on the combinations, if you choose an odd amount (e.g. 2 infantry + 1 artillery = 10 IPCs) then you simply lose the remainder.