I am having the same issue. I have Indesign CS4, version 6.06. A CS4 file was sent to me. When I attempted to open it, a message said "the document xxx uses one or more plug-ins which are not currently availble on your system. Do you want to open anyway?" "SharedContent.InDesignPlugin" is in the box. I clicked OK, then "Cannot open xxxx. Please upgrade your plug-ins to their latest versions, or upgrade to the latest version of Indesign. Please help. Thank you.
A CS4 file was sent to me.
This is most likely not the case. Try the script in http://forums.adobe.com/message/4071273 -- it will tell you what version this file really was saved with.