问个有点难的问题：在表格中如何每隔4行的就变色？

例如第1-4行#F6F4F5，第5-8行#fff，第9-12行#F6F4F5，以此循环。
下面是每隔一行就变色的，如何改呢？
 for (i=1;i<Ptr.length+1;i++) {  
    Ptr[i-1].className = (i%2>0)?"t1":"t2";  
    } 
测试代码：<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>表格行变色
</title>     
    <style type="text/css">
tr.t1 td {background-color:#F6F4F5;}/* 表格交替行第一行的背景色 */ 
tr.t2 td {background-color:#fff;}/* 表格交替行第二行的背景色 */ 
  </style>  
</head>
<body>               <table id="tab" width="726px" style="border-bottom: 1px solid #ccc" border="0" cellpadding="2">               
                        <tr>
                            <td style="color: #F97705; font-size: 14px">
                                xx有限公司诚邀区域加盟商
                            </td>
                        </tr>
                        <tr>
                            <td>
                                发布单位：<span style="font-weight: bold">
                                    xx公司
                                </span>
                            </td>
                        </tr>
                        <tr>
                            <td>
                                简介 ：asdg</td>
                        </tr>
                        <tr>
                            <td>
                                有效期：2009-12-16 11:41:26</td>
                        </tr>
                    
                        <tr>
                            <td style="color: #F97705; font-size: 14px">
                                xx公司诚邀区域加盟商
                            </td>
                        </tr>
                        <tr>
                            <td>
                                发布单位：<span style="font-weight: bold">
                                    xx公司
                                </span>
                            </td>
                        </tr>
                        <tr>
                            <td>
                                简介 ：d</td>
                        </tr>
                        <tr>
                            <td>
                                有效期：2009-6-8 11:41:26</td>
                        </tr>
                    
                        <tr>
                            <td style="color: #F97705; font-size: 14px">
                                xx诚邀区域加盟商
                            </td>
                        </tr>
                        <tr>
                            <td>
                                发布单位：<span style="font-weight: bold">
                                    xx公司
                                </span>
                            </td>
                        </tr>
                        <tr>
                            <td>
                                简介 ：<Lonasdfg Text></td>
                        </tr>
                        <tr>
                            <td>
                                有效期：2009-5-1 11:41:26</td>
                        </tr>
                    
                        <tr>
                            <td style="color: #F97705; font-size: 14px">
                                xx诚邀区域加盟商
                            </td>
                        </tr>
                        <tr>
                            <td>
                                发布单位：<span style="font-weight: bold">
                                    xx厂
                                </span>
                            </td>
                        </tr>
                        <tr>
                            <td>
                                简介 ：asdf</td>
                        </tr>
                        <tr>
                            <td>
                                有效期：2009-1-26 11:41:26</td>
                        </tr>
                    
                        <tr>
                            <td style="color: #F97705; font-size: 14px">
                                xx诚邀加盟商
                            </td>
                        </tr>
                        <tr>
                            <td>
                                发布单位：<span style="font-weight: bold">
                                    xx厂
                                </span>
                            </td>
                        </tr>
                        <tr>
                            <td>
                                简介 ：asd</td>
                        </tr>
                        <tr>
                            <td>
                                有效期：2008-5-6 8:41:26</td>
                        </tr>
                    
                        <tr>
                            <td style="color: #F97705; font-size: 14px">
                                xx加盟商
                            </td>
                        </tr>
                        <tr>
                            <td>
                                发布单位：<span style="font-weight: bold">
                                    xx厂
                                </span>
                            </td>
                        </tr>
                        <tr>
                            <td>
                                简介 ：as</td>
                        </tr>
                        <tr>
                            <td>
                                有效期：2008-3-7 12:41:26</td>
                        </tr>
                    
            </table><script type="text/javascript"> 
var Ptr=document.getElementById("tab").getElementsByTagName("tr"); 
function $() { 
    for (i=1;i<Ptr.length+1;i++) {  
    Ptr[i-1].className = (i%2>0)?"t1":"t2";  
    } 
} 
window.onload=$; 
</script> 
    </form>
</body>
</html>