bill - 7:30 am on Apr 25, 2012 (gmt 0)
Looks like a good thing to update. I've updated a few machines already and can report no issues.
Microsoft Security Essentials 4.0 free PC security software ready for download [zdnet.com]
The 4.0 version has been in beta since late 2011. As ZDNet sister site TechRepublic reported back in December 2011, Microsoft officials said the 4.0 release would include a streamlined interface; a renamed version of the SpyNet service (now slated to be known as Microsoft Active Protection Services); new automatic remediation functionality; and overall improved performance and detection capabilities.