I think I am early in stage 4. But I am 21 years old. I am about 5' 10". I have a beard and chest hair. But the problem is I haven't noticed any growth in my testicles or my penis. My left testicle hangs much lower than my right, but it is also much smaller. It's barely an inch in length.
Having a full beard and chest hair usually indicates being in stage 5. Glancing through your other answers, it appears almost everything points to stage 5. Having one testicle smaller than the other can happen. There are numerous possible causes. Of course, I can't see you to verify this conclusion, but you can see your local doctor and ask his opinion.
Well when I was 13 I used to see an endocrinologist because I had pretty bad gynocomastia. They checked my levels and told me my estrogen levels were too high. But instead of doing anything the doctor insisted I wait it out. I haven't really noticed much development since then, except for hair growth. The gynocomastia has almost gone away. Would it be too late for me to start a hormonal treatment so I can "catch up?" I'm worried I won't be able to have children. I had my levels checked about a year ago and my testosterone levels were still low for my age. So would that make a difference or am I done growing for good?
The question to ask is whether your growth plates have closed, which at 21 is very likely to be true. Once the growth plates close, they cannot be reopened, so any treatment would be too late for growth. A general practitioner can answer this question with an x-ray. If growth in general is done, growth in the penis or testicles can't be encouraged either, though you could check with your endocrinologist to be certain. Sex can still take place, even with a smaller than average penis size. Enjoyment from sex comes from technique, not from the size of your equipment.
Low testosterone would typically indicate that you had damage to your testicles, such as an undescended testicle when you were a child. Having one testicle noticeably smaller than the other might be sign of this problem. The question to ask a doctor today is whether you are fertile. You would need to see a urologist about this question. He will test a sample of your semen and the results will tell you the quantity of sperm you are producing and amount that are active enough to cause pregnancy. If you need help understanding the results, you can write again. As long as you are producing some active sperm, there are things you can do to increase the odds of having a child, but we can talk about those after you know what you are working with.