关于栅格布局困惑

<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://cdn.bootcss.com/bootstrap/3.3.7/css/bootstrap.min.css">
<style type="text/css">
.bg-red {background-color: red;}
.bg-green {background-color: green;}
.bg-blue {background-color: blue;}
</style>
</head>
<body>
    <div class="container">
        <div class="row">
            <div class="col-md-4 bg-red">(1)</div>
            <div class="col-md-8 ">
                <div class="row">
                    <div class="col-md-4 bg-green">(3)</div>
                    <div class="col-md-8 bg-blue">(4)</div>
                </div>
            </div>
        </div>
    </div>
</body>
</html>