Flash Builder 4 does not change the way you would work with .net servers compared to earlier versions. You could either use flash remoting to communicate from swf to the server or you could use web service. For the latter, FB4 has an easy way to connect to web service from flex applications. (Try Data menu connnect to web service) For the latter, you would need a remoting implementation on the server (such as Web ORB) and would need to write remoting connectivity code yourself (using mx:remoteObject).
Thank you much Ram,
Sounds like the "Web Services" option works best for me (I don't have access to make modifications to the server and it's settings). Thank you for your timely answer. I'm off to build my first Catalyst / Flash Builder App!