2 years is not an adult by any stretch of the imagination, although you should notice the most extreme growth spurts during their second year (the year between the time they turn 1 and the year they turn 2). 4' seems good for a 2 year old! As far as I know, boa constrictors reach max size around 5-6 years, unless overfed or power fed, or conversely, when underfed. Power feeding doesn't give you larger animals, just bigger animals sooner, while shortening their lifespan considerably. They can deal with being underfed easier than being overfed. My male is nearly 4 years old and is growing a noticeable amount when you put a tape to him, but is growing considerably slower than before. He only put on 3" in length over an 8 month period of time, where during his second year he was growing 2-3 inches a little less than every month. You should continue to notice "extreme"/very noticeable growth without actually measuring him for another year or so before his growth spurt slows down considerably.