The css can use and redirect to any port you want.
If I understand correctly, your firewall only allows port 4080 and you would like to add a server on port 8080.
You can keep your vip listening on port 4080 and simply configure your service to nat the traffic to port 8080.
You can do this with the following config :
service Myweb
ip address x.x.x.x
port 8080
active
You can then use this service in the following rule
content Myweb
vip address x.x.x.x
add service Myweb.
proto tcp
port 4080
active
Regards,
Gilles.