I read your update and I don't really need to read all the other posts to know the kind of advice you got. I am very often a rebel here and don't really like all this granola/pop psychology gobbledy goop that goes on here.
There is nothing wrong with having expectations that a 4 year old learn manners and respond to other people in a socially appropriate way!
When someone greets your son and he doesn't respond, simply prompt him as to what he should say... "Michael, Mrs. Jones said hello to you, can you please say hello back to her?" "Michael, Mr. Smith said that he really thinks you look nice today, can you please say thank you?".
If your son can't bring himself to respond, then say to the adult, "I'm sorry, Michael is very shy; we are working on it." and continue with your adult conversation. But he may start to respond some and with repeated prodding he will learn to make the appropriate responses.
You are on the right track with what you want from your son and don't let anyone tell you that you are damaging his self esteem by having expectations. He doesn't have to be outgoing, but it is a good idea to learn to be polite. He won't just "develop" manners spontaneously one day, he has to be taught and you are right to start doing it now.