I just turned 16. I have some hair under my arms, a lot of pubic hair, and some hair above my lip. My penis is only about 1 1/2 inches when soft and about 4 when erect. My testicles are not very big and are about 1 1/2 inches long. I do masturbate a lot, and always ejaculate semen. I'm about 5 six which is only an inch smaller then my father and mother, and I weigh 185 pounds. I do experience random erections, and am horny all the time. My questions are: is my penis done growing (if not when will it start), does masturbating stunt penis growth, and what stage of puberty am I at right now?
AnswerRegarding stage, most of what you said points to stage 4. It is an educated guess, but I hope you understand that you would have to see a doctor or someone knowledgeable about growth to get a more accurate estimate. For example "a lot of pubic hair" is actually vague. Someone in stage 3 has a lot compared to someone in stage 2, but less than someone in stage 5. Spontaneous erections generally start in stage 2 and continue for quite a while. Being able to ejaculate semen can start anywhere between stage 2 to stage 4. Having strong sexual urges (feeling horny all the time) is a part of being able to ejaculate and not being used to the feelings yet -- it takes quite a few years. Your testicle size puts you between stage 3 and stage 4. I wrote a calculator that asks more precise questions. Depending on the accuracy of your answers, it will give you a pretty good idea of where you are at in development. See: Tanner Stage Calculator for Boys
Masturbation does not effect penis growth. See: Does masturbation before the penis reaches its full size slow down its growth?
Since you are already four inches long when erect, your penis has already started growing. Boys before puberty don't have penises that long. If you are in stage 3, your penis will still be getting longer. If you are in stage 4, you will get a little bit longer, but most of the growth in increased width of your penis. The typical range for erect male penises is between 3 and 8 inches long, so you are already in a normal range.
At 185 and 5'6" you have a body mass index of 30, which puts you on the boarder between "overweight" and "obese." (Unless you are very muscular.) Being overweight will make your penis appear smaller since part of your penis is inside your groin. Extra fat hides the base of the penis so that the total length is shorter.