I did not talk at 4 yrs old, my parents put me in speech therapy, in fact they came to the house. I'm not sure why I didn't talk but I finally came around and just started talking, i don't think you should worry too much as i seemed to have turned out just fine, in fact i talk just about to anyone who will listen!!
One more thing, I was incredibly shy in grade school and didn't talk except to my very closest friend in fact maybe she needs to interact with other girls her age and become more social, just give her time!
Good Luck, sincerely Laurel94
Selective Mutism is a severe social anxiety disorder. Children who are unable to speak in the home comfortably are said to have "progressive selective mutism" and this is very rare, but treatable. Please note my phrase "unable to speak" rather than does not choose to speak - some professionals think the anxiety is so overwhelming that the muscles in the larynx actually "freeze" and thus prevents the child from speaking. The best site on the internet is "selectivemutism.org" and there you should be able to find answers to see if this description is similar to that your your daughter. Hope this helps ...
I would give the speech therapy some more time before you move on to the next step. It sounds like your child has selective mutism --she can talk, she just doesn't. I would look that up on the web to find out about it.
When my younger sister was an infant, the baby sitter bumped her head on a coffee table while watching us. My sister had a dent in her head, and the doctors told my parents that she might have problems in the future. When she was four years old, she wouldnt talk. My parents had never heard her say a word. Not mom, not dad, nothing. Then one day they heard us in our room playing...and she was speaking full sentences to me. My parents couldnt believe it. All this time, she had been talking to me, I was only one year older, and no one knew about it. Sometimes something happens that puts kids into shock(not speaking) or they get used to the attention that they are receiving due to NOT talking. It sounds like she does talk a little, but maybe she sees how excited you get, or how much you are anticipating her speach that she clams back up.