I am 13 and the Tanner Stage Calculator says I'm in stage 4, but I have no arm pit hair. Is that normal? I only have a 4 1/2" penis that has not grown in length but in width. Does this mean I'm going have a small penis forever? All my friends who are 13 have 6 inches and are like adults. They all have a thick strips of hair under their navel too and I don't. Is that normal?
Is wearing briefs at this age still weird?
Everyone develops at their own pace and each person is unique in regards to their shape. The average boy hits his growth spurt when he is 13, but the range is normally between 10 and 17. The stage lasts about two years. It is during stage 3, the rapid growth stage, that the penis grows in length.
I'm mentioning all of this to point out that either you have very few friends or you are not being accurate in your statements. There are boys around you who have not even started changing, some who are nearly done, but most are right in the middle of changing.
The calculator only gives estimate based on your answers. Inaccurate answers can give inaccurate results. Since I don't know you or have seen you, I cannot comment on whether the results are accurate or not.
Whether you will have much body hair depends more on the genes that you inherited. For example, people from far eastern countries tend to have very little body or facial hair. When that hair develops also varies between people. Some boys develop hair later than others.
What you choose to wear for underwear is entirely up to you. You wear what you find comfortable.