BETA4 provides a sizable number of bug fixes and minor missing features that have accumulated since BETA3. BETA3 is running on 800+ known sites with very little issues and we expect BETA4 to be the last beta version before the release candidate. Be sure to test it thoroughly if you want to deploy it on a production site, and please do provide feedback (both good and bad!) in the issues queue. See #1172646-22: Subscriptions for D7 for details.