Is 4 months too soon for a puppy to be in heat?

No, a 4-month-old puppy is not old enough to be in heat. The average age for a female dog to experience her first heat cycle is between 6-12 months of age, depending on the breed. Some smaller breeds may experience their first heat as early as 4 months of age, while larger breeds may not experience it until they are over a year old.