I am completely lost. The website I check for CSS-defaults is always http://www.w3.org/TR/CSS2/sample.html (I don't trust any other site)
And it seems they missed the font-size of h4.
Sorry to bother you guys for this, but it is important and I don't know how to check it using the tools like firebug or one of those...
It's not there, because it's using the implied default font-size of 1em (which also happens to be the average between the values for <h3> and <h5>). As others have noted, though, this may not be true for your browser, and you should really set it yourself if you want to make sure.