Tell... 4 Year olds are SOOO smart! I have been so close to my nephew who is 4 and amazed at this age how they go to learning something new every day like a new word or how to walk, to really being intelligent and smart! Shocking at times even...
He's at the age where you'd do more hurt to trick him then to prepare him. Hype it up about a reward afterwards. He may even remember shots in the past, remind him how he made it through that just fine (if he didn't he wouldn't be here today for the next one, meaning no matter how hard that one was or how bad an experience that one was it only lasted a few minutes at the most. And if it wasn't that bad and he does remember it then it wouldn't be any worse this round...).
Tell him the nurses called to make sure he was ready for this doctor's appointment and can't wait to see him, can't wait to give him his sucker after he's all done, something that really makes him feel special!
I'm not an expert by any means but I think they are too smart at this age for tricks. It might cause more harm like you said... He needs to respect and feel incredibly comfortable with doctors and nurses etc.
Give him the respect you would want. Crazy to consider of a 4 year old but he's not a little boy any more mommy! That means even more fun stuff in life! Promise him a trip to the library for his favorite book or a trip to McDonalds for a happy meal, or a trip to the toy store for a new toy. Depending on which direction you wanted to take it, and what he'd really love... I wouldn't promise him the world though, its just a shot, but does need some cushioning to deal with the situation.
Hope that helps! I love to share my two cents, whether you use it or not is all up to you! GOOD LUCK!!! :o) ~T.~