How can I setup subdomains to point to the same ip address at different ports?

I have a linux server that is running 4 different instances of a game server. To connect to the server I have my router setup to forward 4 different ports(7707, 7709, 7711, 7713) to the correct machine. What I would like to do is somehow setup aliases to allow me to specify 'srv1.mydomain.com'(7707), 'srv2.mydomain.com'(7709), etc. to point to the correct ports. I have a godaddy domain name and account, but CNAME's don't allow for directing to different ports. I don't believe iptables will allow me to filter by domain name either, since it functions at the tcp level at it's highest. Does anyone have any idea what I could do?