I'm not familliar with FrontPage 4.0 but look to see if there is a way to 'edit raw' the top frame. Then insert the text
width="100%" exactly as below.
Code:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<title></title>
<base target="contents">
</head>
<body bgcolor="#99CC00" link="#008000" vlink="#008080" alink="#FF0000">
<p align="center"><a href="index.htm" target="_parent"><img border="0" src="images/mainlogo.gif" align="top" width="100%"></a></p>
</body>
</html>