Hi, Kaninchen. Bipolar can manifest at a very early age, and is often far more severe than the adult version. And owing to the fact you have bipolar, it is wise for you to be vigilant.
But there is this complication: 4-year-old behavior is no picnic, but it may appear to be worse than it actually is from the perspective of a parent with bipolar. Our population tends to reach its breaking point very quickly and we often see things as worse than they actually are.
So you need to analyze how your own illness is affecting how you view your child's behavior.
I recall talking to Joseph Biederman, the pioneer in diagnosing and treating kids with BP. He actually says the diagnosis is fairly easy to make, as we are talking about extremely frightening behavior such as kids jumping out of moving vehicles.
Keeping this in mind, if you haven't reached this "fright point," maybe it's best to suspend judgment for now, but while remaining vigilant. The last thing you want is to be medicating your son for an illness he may not have. I'm sure you thought of this, already, and I know you have your son's best interests in mind.
Please do not interpret my cautious view as someone who doesn't believe you. I've listened to hundreds of parents with kids with bipolar. I've heard their heartbreaking stories about how others don't believe them, how others blame the kids' behavior on bad parenting and the like.
Believe me, I know kids can have bipolar, just like they can have cancer and other terrible illnesses. For support, I suggest checking out www.bpkids.org. Hope this helps ...