How can you differentiate between oppositional defiant disorder and just "bad" behavior in 4 year old boy?

What's "bad"? Odd is a description of how a child interacts with others by being negative and resistant. "bad" is a moral judgment that can be associated with lots of behaviors. Unacceptable behaviors needed effective correction, hopefully from loving parents who see the good in their child even when the behavior is bad.
Difficult Question. This is a very difficult question. The best answer, i think, is comparing the behavior with his peers. Is it much, much worse?Also odd tends to occur with everyone, everywhere-home, school , at friends etc.Bad behavior usually responds to authority figures other than parents such as teachers etc.
Severity . Oppositional defiant disorder means that there is the persistent presence of severe defiance and temper outbursts for at least 6 months. The diagnosis should be made only rarely under age 6 due to the wide range of normal behavior seen in preschool kids.
Kids aren't bad. Telling a kid he or she is bad can be a self fulfilling prophesy. Check out the kindle book, parenting the kids according to a child psychiatrist. For only $5.95, and some effort, you don't have to have a "bad" kid.