> How do I limit to <h4> elements following the current <h3> 
> element, up until the next <h3> element?

In 2.0, <xsl:for-each-group group-starting-with="h3">.

In 1.0,

following-sibling::h4[generate-id(preceding-sibling::h3[1]) =
generate-id(current())]

Michael Kay
http://www.saxonica.com/