irule for port redirection not working

Your redirect is telling the client to try again on the different URL (and on port 4000). Assuming this web app is only listening on port 4000, and you want clients to come to it on port 80, you probably don't need an iRule at all. Just create a port 80 virtual server that pools to servers listening on port 4000. A standard virtual server will have port and address translation turned on by default.