I have a few questions that I’m confused about. First, you said that during stage 4 the penis grows in girth. I know it varies between people, but generally, is the growth noticeable? Also, because I’m larger than average in girth, would that mean I’m going to grow less? If I’m in stage 3.9, shouldn’t my growth spurt have stopped?
The typical growth pattern for males is for the penis to grow mostly in length in stage 3 and mostly in girth in stage 4. The circumference of the penis still increases during stage 3, but generally not as much as the length. Similarly, the length of the penis can still increase during stage 4, but generally not as much as the girth.
Most people notice the increase in length. Fewer people pay attention to the increase in girth simply because the amount of change is not as dramatic as the change in length.
“Larger than average” is a vague phrase because I don’t know what you are comparing your girth to determine an average. I don’t know of any study that specifically looked at girth changes during adolescence, other than to determine broad generalizations. Thus, I don’t know if a person who already has significant girth in stage 3 will continue to gain more girth in stage 4.
Stage 4 is the time when your rate of growth in height slows down. You won’t actually stop growing in height until you reach stage 5. The Tanner Stage Calculator tries to narrow down your stage to within 0.3 of your actual stage. Thus, a 3.9 means you are somewhere between 3.6 and 4.2. If the estimate is a little high, then you’ll continue growing fast for another 3 to 6 months. If it is a little low, then you’ve already started slowing down and your gains in height won’t be as dramatic.