it's most likely, that the document you want to open is not an CS4 file.
If you are absolutely sure, after running the script, that the file is CS4 and you still cannot open it, what is the exact error message you are seeing?
It could be a missing 3rd-party plug-in that was used by the creator of the document that hinders you to open the file: Then you have to ask the creator of the file to provide an IDML file instead of an InDesign document and procede with opening the IDML with InDesign CS4.
I'd try Jongware's script first to find out anything about the version of the document.
What happens if you click OK after getting this message?
if i click OK after this error, i won't be able to edit the file.
Did you try the script? Does it report CS4?
If it reports as CS4, have you checked to be sure the listed plugins are installed and activated? You can launch the CS4 Extensions Manager from the Start Menu to check.
Actually i don't know how to run the script, can you please help me out.
The Online Help:
http://indesignsecrets.com/how-to-install-scripts-in-indesign.php (and if the script is listed in a post:
Finally, in the post itself: ".. unpack, and put "IDentify.jsx" to the folder where you put all of your User scripts."