I am currently 18 years old, 250 lbs., and 6'4" tall. I am semi-active, I don't play many sports but I walk a lot. I have read that durring puberty many changes occur. I first started masterbation (without ejaculation) at age 7 or 8. I was 13 the first time I ejaculated. Both of wich I am told is normal. However, I just started really growing about 2 years ago, in that time I grew between a foot and a foot and a half. Although it has dropped a little, my voice range is still high tenor and up. And the third thing is that my penis has only grown 1/2 and inch since I was 13, making it 4 inches long.

Is this normal? Is there anything I can do to encourge growth?