I would like to weigh him, but i cant justify the cost of an accurate scale just to satisfy my curiosity. If i were planning on breeding, or had a few snakes of different ages whose growth i was tracking, then i would surely invest in one.
regarding the site, try taking some pictures of a rope and experiment with different angles and different "rulers" to see what gives you the most accurate results. One thing i o is to make sure to follow his centerline as closely as i can, not cutting any corners and using as many points as needed. Its a little tedious, but only takes few minutes really.
Which is your 4-year-old?