M.B.
Please don't be so quick to self diagnose your son with something that will stay with him the rest of his life.
My son showed "signs" of ADHD too and of all sorts of other things if I looked for it (and at one time many years ago, I did). I thought my friends son at age 4 was ADHD too. Neither of them were. They were just boys and they were actually age appropriate behaviors.
I'm sure you've been pulling your hair out because he likes to play rough, he's mouthy and maybe talking back to you already. You've probably googled symptoms or signs of ADHD, maybe even Autism too. It is so easy for us to look for a problem with the internet so readily at hand. I've done it as I'm sure most of us have. Even when my daughter was a newborn, she sneezed and I immediately googled "newborn sneezing". We worry about our kids as mother's only do and we want to do what's best for them.
Just now when I googled "boys development behavior" I got this http://www.brainy-child.com/article/boys-behavior.shtml
I highly suggest you read it.
With my son, my ex's new wife pushed, when he was age 9 to have him tested. She had only known him for about 3 months and this was the precurser to a custody case. She was convinced he had ADHD. She had two children who had been diagnosed at an early age with Aspergers. My son was tested and he was and is fine. So she changed her "diagnosis" to "oppositional defiance disorder". When he was tested for the Gifted and Talented program the following year, I was a little surprised that in the flyer they sent home about recognizing gifted children, that a lot of the same "ADHD" warning signs were also signs of high intelligence in children. At this point, I just new my son was a boy. :)
As a mother of a boy and having known so many boys now through my various activities in school, Scouts and church, I've seen that boys are definitely more hyper, more agressive and after doing a lot of my own research (yes I google too!) I'm more convinced than ever that we are over medicating our boys. I might be wrong, I'm definitely not a doctor but a lot of professionals have the same opinion.
I suggest you talk to your pediatrician about his development and especially that you also look into some of the following books.
The War Against Boys - You can read the first chapter here http://www.nytimes.com/books/first/s/sommers-war.html
And Raising Confident Boys: 100 Tips for Parents and Teachers- You can read part of that book here http://books.google.com/books?id=MhvK3ZZO3W4C&dq=rais...
I've learned to celebrate my son for the boy that he is and as he starts to grow into a young man, he excels in school, he's involved in various musical programs in and out of school. He's not extremely social but he has a few extremely close friends. He's confident about who he is and he has definite goals about where's he's going.
Anyhow, I hope I've helped you in some way and good luck to you. :)