Could you answer if it is bad for a 4 month old to get chicken pox?

Maybe: Chicken pox can be deadly, but in most cases is tolerated fairly well. Usually it is milder in babies and they get fewer lesions. It is usually more severe in adults. Some babies that get chicken pox will not develop an adequate immune response and will get it again when older, if not vaccinated.

Usually mild: Infants born with it (as well as their mother) can suffer fatal outcome unless aggressively treated.Those whose mother had it as a kid and passed on some of her own immunity, do better. They may become vulnerable to chickenpox at a few months, but this is usually mild because of mom's partial protection.