Not "at least", more like "at most". It's 4months + 4months. And you have to "willing-fully" accept the second 4 months (but they can fire you if you don't).
The "faute grave" you are referring to is the "faute lourde", for which the employee voluntarily caused harm to the company.
A "faute grave" is more like a big mistake, harassment, misbehavior, etc.
Basically, you can fire anyone you want if you pay for it. But you'll have to pay, and if the employee is not happy with what they receive, they can go to court (which is very long, most of them won't).
The "faute grave" you are referring to is the "faute lourde", for which the employee voluntarily caused harm to the company. A "faute grave" is more like a big mistake, harassment, misbehavior, etc.
Basically, you can fire anyone you want if you pay for it. But you'll have to pay, and if the employee is not happy with what they receive, they can go to court (which is very long, most of them won't).