I created a simple Silverlight project based on the provided description and snippet code. However, I was unable to reproduce the reported problem in my local end (as shown in the attached video). It adds to the list box and clear out the UXTextBox.

Please have the attached SL project evaluated on your end and let us hear your feedback.

UXTextBox is a fundamental input control generally used to obtain input data from users through input devices such as keyboard. Similar to native text box control (assume that the Text property is bound to a property in View Model – let’s name it as BarCode property), the set accessor of the property will be invoked when user tabs out of UXTextBox or press Enter key.

This default behavior matches with your requirement, to update the list when the user tabs off the textbox (and/or when 4 characters have been entered). Please try to use following snippet code in your View Model.

public string BarCode
{
    get { return _barCode; }
    set
    {
        if (_barCode != value)
        {
            _barCode = value;
            OnPropertyChanged("BarCode");
            if (!string.IsNullOrEmpty(value))
                AddBarCode();
        }
    }
}
private void AddBarCode()
{
    if (!string.IsNullOrEmpty(BarCode) && BarCode.Length == 4)
    {
        ContactsData.Add(new Contact { Id = BarCode.GetHashCode().ToString(), Name = BarCode });
        SetFocusBarcode();
    }
    else
        MessageBox.Show("Please enter valid BarCode character (4 characters)");
    Utility.ExecuteTimeOut(0.1, () =>
    {
        this.BarCode = string.Empty;
    });
}
private void SetFocusBarcode()
{
    ISFocusManager.SetFocus("EnterBarCode");
}

This should help.