Add more than 4 social buttons to TopBar?

Those are just rendered as HTML.

So you can use the new suffix PHP to add additional images/links as you need:

echo '<a href="http://twitter.com/#!/wordpress" target="_blank"><img src="http://domain.com/wp/wp-content/plugins/wp-topbar/lib/icons/PNG/twitter.png" style="height:23px; vertical-align:text-bottom;"/></a>';