зверек,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function() {
var a = $("em:first"),
b = $("em:last"),
c = [];
a.nextUntil(b).add(a).add(b).contents().each(function() {
var a = $.trim($(this).text());
a && c.push(a)
});
a.html(c.join("<br>"));
a.nextUntil(b).add(b).remove();
alert($("body").html())
});
</script>
</head>
<body>
<em>
знач.1
</em>
<br>
<em>знач.2 </em>
<em>
<br>
знач.3
</em>
<br>
<em>знач.4</em>
</body>
</html>