MSXML4 is going to be kill bitted sometime between October and December of this year. That means it won't be accessible from Internet Explorer and that web sites will need to transition to MSXML6.
Feedback on this change can be sent to msxml4@microsoft.com.
Microsoft's XML Team has more information on this change, the history of MSXML, which version to use, and how Internet Explorer uses kill bits to disable old ActiveX components.