title: How do I place four buttons evenly with AutoLayout?
body: <p>There are four circle <code>UIButtons</code> in <code>UIView</code>. Their layout should look like this:</p>
  
  <p>｜－ button 1 - button 2 - button 3 - button 4 －｜</p>
  
  <p>I tried some ways with AutoLayout but failed. 
  Can someone give me some advice or demo about how to build this layout? </p>