title: Insert hyphen automatically after every 4 characters in a TextBox
body: <p>I want to implement some thing that when a user enters 4 characters then a hyphen should be added to the text and then again user enter 4 characters and then again hypen should added automatically in WPF.</p>
  
  <p>Note </p>
  
  <p><strong>"I want to implement this behavior when user is typing in the text box (not after textbox has lost focus) beacause the later is very easy to implement"</strong></p>
  
  <p><strong>Using MVVM model, therefore code behind of Dialog should be empty</strong></p>