Interesting...
Removing the gutter in Twitter Bootstrap's Default grid, that is, 940px wide. And that the default grid has a 940px wide container and has the bootstrap-responsive.css in it's stylesheet.
If I got your question right, this is how I did it...
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Stackoverflow Question</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="">
<meta name="author" content="">
<!-- Le styles -->
<link rel="stylesheet" href="assets/css/bootstrap.css">
<link rel="stylesheet" href="assets/css/bootstrap-responsive.css">
<!-- HTML5 shim, for IE6-8 support of HTML5 elements -->
<!--[if lt IE 9]>
<script src="assets/js/html5shiv.js"></script>
<![endif]-->
<style type="text/css">
#main_content [class*="span"] {
margin-left: 0;
width: 25%;
}
@media (min-width: 768px) and (max-width: 979px) {
#main_content [class*="span"] {
margin-left: 0;
width: 25%;
}
}
@media (max-width: 767px) {
#main_content [class*="span"] {
margin-left: 0;
width: 100%;
}
}
@media (max-width: 480px) {
#main_content [class*="span"] {
margin-left: 0;
width: 100%;
}
}
<!-- For Visual Aid Only -->
.bg1 {
background-color: #C2C2C2;
}
.bg2 {
background-color: #D2D2D2;
}
</style>
<body>
<div id="wrap">
<div class="container">
<div class="row-fluid">
<div class="span1 text-center bg1">01</div>
<div class="span1 text-center bg2">02</div>
<div class="span1 text-center bg1">03</div>
<div class="span1 text-center bg2">04</div>
<div class="span1 text-center bg1">05</div>
<div class="span1 text-center bg2">06</div>
<div class="span1 text-center bg1">07</div>
<div class="span1 text-center bg2">08</div>
<div class="span1 text-center bg1">09</div>
<div class="span1 text-center bg2">10</div>
<div class="span1 text-center bg1">11</div>
<div class="span1 text-center bg2">12</div>
</div>
<div id="main_content">
<div class="row-fluid">
<div class="span3 text-center bg1">1</div>
<div class="span3 text-center bg2">2</div>
<div class="span3 text-center bg1">3</div>
<div class="span3 text-center bg2">4</div>
</div>
</div>
</div><!--/container-->
</div>
</body>
</html>
And the result is..
The 4 div span with no gutter will remain spanned for Small tablet landscape (800x600). Anything size smaller than that will collapse the 4 divs and it will be stacked vertically. Of course you will have to tweak it to fit your needs.