Users interested in downloading and testing the software have to sign up for the beta test and provide either a Windows Live ID or Connect ID.
According to Microsoft, the new version has received a new "protection engine" with better detection and cleanup features. MSE4 will automatically remove malware and not require user action in cases of "high-impact malware infections". The user interface has been slightly changed and further simplified. Microsoft says that the new version is optimized to make better use of background resources, won't use as much memory and won't degrade a PCs performance during scans and cleanups.
Some users may notice that the current version of MSE is 2.1. Apparently, there will not be a version 3 of the software.