How can I center buttons inside a DIV so there's the same space right and left of the four buttons?

How can I center buttons inside a DIV so there's the same space right and
left of the four buttons?

I have the following HTML:
<div class="clearfix">
<div>
<button>a</button>
<button>a</button>
<button>a</button>
<button>a</button>
</div>
</div>
The buttons appear to the left and leave a space on the right of my outer
DIV. How can I make it so the buttons center themselves leaving a space on
the left and the right that is equal?