Update version number in header

Author: mrmrs

docs/themes/opacity/index.html

@@ -30,7 +30,7 @@
     <a href="/" class="dib f5 f4-ns fw6 mt0 mb1 link black-70 dim" title="Home">
       Tachyons
       <div class="dib">
-        <small class="nowrap f6 mt2 mt3-ns pr2 black-70 fw2">v4.0.1</small>
+        <small class="nowrap f6 mt2 mt3-ns pr2 black-70 fw2">v4.0.4</small>
       </div>
     </a>
   </div>