Gone? Or just you can't open it any longer in CS4?
Can the person who worked in CS6 prepare the files for the printer? Will they export to .idml for you so you can open it in CS4 and proceed?
Have the person who did the CS6 work export a PDF and be done with it.
Saving back via IDML is going to cause problems.
Not sure. His InDesign crashed after he tried to save the changes. He could
no longer get it to open at all. We re-did the paper, but it would be nice
to know how to fix the problem for the future!
The answer to fixing the problem is to not mix versions. Anyone working on the files should be using CS4, if that's what is installed onthe school machines, or the school should upgrade to CS6.
It's possible to export files from CS6 to interchange format (.idml) and open those .idml files in CS4, but the chances of different line endings or lost content due to the use of an unsupported feature in CS6 are pretty high.