If you want to always run the latest and greatest software, Microsoft is giving you the chance with the Microsoft Security Essentials (MSE) prerelease download.
Microsoft Security Essentials prerelease is offered for those that would like to take part in running the most current revision of the security product as long as you agree to provide feedback. During installation, you will be automatically enrolled in Microsoft Error Reporting, the Customer Experience Improvement Program, and the Microsoft Active Protection Service. Each of these built-in functions sends data to Microsoft on usage. To get an understanding of what exactly Microsoft receives from your participation, take a look through the Microsoft Security Essentials prerelease privacy statement. These same functions are built into the regular releases, but you can choose to opt out.
To opt out of the prerelease program, you have to go through two steps. First, you uninstall the application. Then, you have to jump out to a link on the Microsoft Connect site to quit the program.
There's one other, more manual way to provide feedback on the prerelease product, and that's through the web-based forums located on the MSDN site HERE.
To start your participation in the program, download the installation here: Microsoft Security Essentials Prerelease
It should be noted that any antivirus software should be uninstalled prior to installing the Microsoft Security Essentials prerelease, this includes any other versions of MSE. More information can also be found on the official Microsoft Connect landing page for MSE prelease.
Once installed, Microsoft Security Essentials prerelease will show version 4.6.0205.0.