Hi, Scott. It's not my place to second-guess the boy's pdoc, as every individual and every situation is unique. But you are entitled to ask very hard questions and demand satisfactory answers from the boy's psychiatrist. And if you are not satisfied to fire the shrink.
As a general rule, 20 mg of Zyprexa is very high, even for an adult. And two drugs of the Zyprexa class is very unusual. Both Zyprexa and Risperdal are what they call atypical antipsychotics. These are extremely powerful meds and can have very pronounced side effects. And kids are often more prone to side effects than adults.
There may be a valid reason for the double-dosing, but if the pdoc fails to provide you with one then get another opinion.
When someone is diagnosed this early, it is called early-onset bipolor disorder. The experts regard this as far more severe than the adult variety. Untreated, these kids tend to cycle extremely rapidly, their manias can rage out of control for hours on end, their sleep is messed up, and they can be extremely grandiose.
Believe me, their behavior is not usual four-year-old behavior. This is why meds treatment tends to be a no-brainer. But it needs to be smart meds treatment.
You talk about adoption. I don't know the kid's circumstances. But perhaps in a foster setting, he hasn't received the medical and psychiatric treatment he deserves. Institutional abuse and neglect is fairly common in foster settings. Maybe the home he was in erred on the side of over-sedating him so he wouldn't wouldn't disturb the other kids (or his caretakers). But this is only a wild guess.
Certainly a loving home environment will remove a lot of the stresses that feed his illness, but he will probably still need the meds. If the boy had cancer, you would take him to a pediatric oncologist. For similar reasons, a child and adolescent psychiatrist is the specialist for your boy. If one is unavailable, then a psychiatrist with experience and some extra training in working with kids.
Please be assured your boy has every chance of enjoying childhood and of growing up to have a rich and happy and productive life.
Must-read book: The Bipolar Child by Demitri and Janice Papolos.
Also check out this article:
http://www.mcmanweb.com/article-30.htm
Check out my bipolar articles and blog on HealthCentral.