Here's the link to the Skype 4.0 Beta for Windows. This is a good link to bookmark since it provides the early trial users of a home to find the latest versions. Once the pool is full its likely there won't be any more access to the beta.
Since Skype has an auto-update notification feature once you have installed the beta you'll likely be notified of new builds that come out. That said, you may want to refer to the page just in case.