Mihai Bocsaru, a long-time ProNet member and all around MT enthusiast has been doing a great job of running a public demo of MT4, which he's updated for the release of Beta 2. You can check out his demo site or log right in to MT4 Beta 2 with username "demo" and password "demo".
Now, please note: This isn't an official demo site, this is just something Mihai's done to support the community. So be nice to the demo! But if you haven't had a chance to install the new version yourself yet and just want to see the shiny new goodies, you're all set. Thanks, Mihai!