HOW OLD IS MY BOA CONSTRICTOR ?

First off, they don't stop growing because they are kept in a small cage. It grows based on feeding, not cage size. Your cage is big enough for a male and for all but the largest of females although 6 feet by 2 feet would be better for adults. There is no way to tell the age based on its size. It's either a yearling that was fed heavily or a two year old that was fed normally. Boas don't grow real fast like some snakes such as Burmese pythons, so at four foot, I'm guessing it is more around 1 1/2 to 2 years old, but as I mentioned, it could be a year old but only if it was fed a lot.