In order to draw the PNG, the browser has to decode it, including the transparent pixels. I'm also pretty sure (though I haven't tested this) that the overhead of painting four smaller images is negligible in comparison to the cost of blending the entire center area.

Many sites are very bad about this, e.g. this sprite sheet from linkedin: https://static.licdn.com/scds/common/u/img/sprite/sprite_glo... which, when decoded, takes up ~9MB of almost entirely empty memory. Check out the "Optimizing Web Content in UIWebViews and Websites on iOS" talk on https://developer.apple.com/videos/wwdc/2012/ for more info about not wasting memory on mobile web sites.