By tagging android, I think you are discussing about android editText, is so you can do it by listening the TextChangedListener,
EDITED: for backspace
editText.addTextChangedListener(new TextWatcher() {
int len=0;
@Override
public void afterTextChanged(Editable s) {
String str = editText.getText().toString();
if(str.length()==4&& len <str.length()){//len check for backspace
editText.append("-");
}
}
@Override
public void beforeTextChanged(CharSequence arg0, int arg1, int arg2, int arg3) {
String str = editText.getText().toString();
len = str.length();
}
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {
}
});