Problem registering sql server

I am attempting to connect to a SQL Server across the Internet, when I attempt to register the server through the Enterprise Manager using the IP provided by the web hosting provider, I get the following error :"SQL Server does not exist or access denied" , I was told by them that I should change the default port from 1433 to 4000. I don't know where in SQL Servre i could make this modification. Anybody knows how?

I have 2 instances of SQL Server, the default one and another named instance, I am using the named instance, the default one has problems & is not working, so changing this port thing would be related to the instance I am working on or to the SQL Server as a whole?

Thanks in advance.