load balancing 4 servers

Pretty straight forward. Create two pools: one with all four and the other with the two specific ones. Assign the pool of 4 to the virtual server and employ an iRule something like this:

when HTTP_REQUEST {
    if { [string to lower [HTTP::uri]] starts_with "/arch/enj" } {
        pool special_pool
    }
}

You could also use a switch:

when HTTP_REQUEST {
    switch -glob [string to lower [HTTP::uri]] {
        "/arch/foo*" -
        "/arch/bar*" -
        "/arch/enj*" {
            pool special_pool
        }
    }
}

And depending on BIG-IP version you could also skip iRules and do all of this in CPM (LTM policies).