So I just used calibre's Tweak Book (thanks again for pointing that feature out to me) to explode the MOBI file and you're totally right -- *kindlegen* is adding a ghost <div> before the <h4> tag with page-break-before: always styling hard-coded to it. I know that's not your code but have you seen anything like that? It even turns HTML like
<h4 style="page-break-before: always">Foo bar</h4>
to
<div style="page-break-before: always"></div><h4 style="page-break-before: always">Foo bar</h4>
And when I remove that style, it just goes away completely.