Microsoft's "What's New" for v4 includes:
- Enhanced protection through automatic malware remediation: The Beta program will clean highly impacting malware infections automatically, with no required user interaction.
- Enhanced performance: The Beta includes many performance improvements to make sure your PC performance isn’t compromised.
- Simplified UI – Simplified UI makes Microsoft Security Essentials Beta easier to use.
- New and improved protection engine: The updated engine offers enhanced detection with cleanup capabilities and better performance.
When Microsoft sent me the invitation to download I downloaded and installed it on my "BenInTheDen" laptop. This is my workhorse PC and it is exposed to lots more sites and programs than all my other PCs put together. After installation I let it update the signatures and then requested it run a full scan.
It found and removed 2 instances of malware that had not been found by the current Security Essentials. That's the last bullet above.
I'm now running it everywhere. You can download the beta version at Softpedia. I suggest you do that right away.