Redirect to a custom error page when server response is matched with any specific code

Can you try this?

Also, you should consider HTML encoding the host and URI values in the iRule to protect site users from XSS.

You can do this using string map:

when HTTP_REQUEST {
    set hostvar [HTTP::host] 
    set urivar [HTTP::uri] 
    set delay 4
} 
when HTTP_RESPONSE { 
    if { [HTTP::status] == 404 } {
        HTTP::respond 200 content " Unfortunately, your request for ${hostvar}${urivar} casued a 404 error. After 4 seconds, you’ll automatically be redirected to our home page. If you feel you’ve tried a valid link, please contact webmaster@sample.com. Sorry for this inconvenience. " "Content-Type" "text/html"
    }
}