K.,
I had EXACTLY the same thing happen to me with my kids! I had an outgoing, happy, full-of-energy son who I was afraid would go home with a stranger and a daugther who wouldn't even go to her father! And who wouldn't talk to ANYONE and didn't even smile in public to the point where people would ask me if she EVER talked or EVER smiled!
Just so you know, she is 20 years old now and a college junior and a very confident, intelligent, remarkable young woman who isn't necessarily shy, but not loud, outgoing and boisterous either. She is organized, she is a logical thinker, and very strong academically. But I remember 4yo being an especially tough year with her. This is what I did.
I played a game with her. It was a game of catch with a small ball. I asked, "When I throw you the ball, what do you do?" She said, "Throw it back." I said, "Having a conversation is like playing catch. When someone throws words to you, you need to throw some back." Then we would practice as we threw the ball back and forth. So I'd throw the ball and say, "Hi, Anna, how are you today?" and then I'd coach her to answer me as she threw the ball back. If she didn't know what to say, I'd help her to know she was to say, "I'm fine, and how are you?" and we'd keep playing catch and practicing our conversations.
Then, when we were going out, I'd remind her about how she was supposed to play "catch" with words and encourage her to do so when someone spoke to her. If someone spoke to her and she hesitated, then I'd gently remind her by saying, "its your turn to throw." It was kind of our private little thing we had between us and the other person didn't need to know what it was about. If she responded politely to the other person, I'd be sure to praise her when we were alone together and tell her how happy I was that she was learning the art of conversation.
Also, I let her know without a doubt that I didn't mind her being shy, but that she was NOT allowed to be rude. Meaning, that I didn't mind if she didn't initiate conversations with others, but if someone spoke to her, I expected her to answer politely. I don't remember how I punished her exactly, but I know that she was punished if she was rude to anyone. I don't think that I had to punish her very many times because she was the kind of child who never wanted to displease me and reacted strongly if I said that I was unhappy with her.
Another thing I had her do during her childhood, as she got older, was to make her ask for things herself. As in, if we were in McDonalds and she needed ketchup for instance, I would tell her to go ask the lady herself. If she hesitated, I would say, "Well, I guess you don't want ketchup." I would not cover for her, and I would not let her outgoing brother do it for her either.
And during her pre-teen and teens, I had her call on the phone to ask for things like store hours, information, directions, prices. During her senior year in high school, I had her call to make her own doctor appts. and fill her own prescriptions, showing her insurance card and answering questions, etc.
For the shy child, it is difficult for them to overcome their shyness, but as we prepare our children to live in the real world, learning these essential skills are necessary for independence. My shy one is confident in her ability to meet, greet, ask for assistance of her professors when needed, apply for jobs, get dr. appts, get her car fixed, and generally speak up to meet her own needs. It is a long process, but with lots of coaching, love and patience, you can help her to find her voice.
Hope this helps,
L.