There's no need to tell him about it before, especially if it's going to cause days of questioning (or in some kids, anxiety). Tell him a little before you leave. Or pack up to do a few errands and head for the hospital after picking off a quick one like driving through the bank. Typically we wouldn't tell most 4 years olds every detail of our weekly routine in advance so there's no need here.
Most of us worry about the negative self image issue in response to the evaluation and I think the vast majority of the time it winds up being a non-issue. I'm no expert, but I suspect the two key themes of the ODD personality (My way of thinking is the right way, I'm in charge) provide a cushion of protection to many kids in this.
Especially when kids are young and you really don't have any answers, I favor answers that are broad, that put it into the positive, and that make the issue seem like just another medical issue we see a doctor for.
Broad (include both positive and negative traits)--We're going to a doctor to learn more about you: we'll probably talk about how smart you are, how funny you are, what makes you angry, what you're good at, what kinds of things you like to play with, etc.
Positive--This doctor helps families. We're going to talk to him to see if he can help us have less angry time and more time for fun things like _________.
Doesn't single them out--I distinctly remember answering "Why do I have to see Dr. X today?" like this: Everyone in our family has reasons they need to make some extra trips to the doctor for: Dad sees Dr. Z because he's had a bump on his arm, Mom makes lots of trips to Dr. N' because of her allergies and ear problems, easy child Jr. makes trips to the dentist because her teeth are shaped like V's inside and she gets lots of cavities, easy child Sr. makes trips to the hospital to get his blood tested because he's been so tired, and you're going to see Dr. X because you've been more scared to go to school than the other kids in your class.
Sometimes there is no answer, you don't know, or the time isn't right for answering--Don't be afraid to say "I don't know" or "Why don't you ask the doctor the next time you see him".
Hope this helps.
SRL