[iRule, URLSLB] How to change destination ip address(VIP).

To answer your question as you asked it - assume you have.....

a virtual vs_www.aaa.com_http with pool pl_www.aaa.com_http, and a virtual vs_www.bbb.com_http with pool pl_www.bbb.com_http

when HTTP_REQUEST {
     This iRule is applied to vs_www.aaa.com_http
    if {[HTTP::path] eq "/test"} {
         Divert to other virtual
        virtual vs_www.bbb.com_http
    }
}

Note that once the destination is NATed to 2.2.2.2, sent to the 2nd VIP, and a pool member chosen, the destination address will again be NATed to either M3 or M4, so I'd agree with afedden that you are better off selecting the final pool directly from the first VIP ie

when HTTP_REQUEST {
     This iRule is applied to vs_www.aaa.com_http
    if {[HTTP::path] eq "/test"} {
        pool pl_www.bbb.com_http
    }
}