I am having the same problem.  New install of 5.1.1, have un-commented the add port , but when I try to go to the logon page I get the 404 error.  This is very frustrating as  I too use port numbers other than 80 for development.  Hope someone comes up with a solution for us. 

HSharma,  I have search this forum and the Internet in general and have not been able to find any solution other than using host headers, which is a pain since I do not have control of the DNS.  I also looked up under the bug reports, but could not find a specific issues with the ports, so I am not sure what is going on.  It would be interesting if someone with a maintenance subscription for DNN could check on this for us.

I've managed to get it to work by adding the the port number directly into the DB. <add key="UsePortNumber" value="true" /> is commented out by default in the web.config, Eric make sure you have uncommented the line.