Trying to open CS5 in InDesign CS4
Any suggestions - How do I update plugins or tell whether they need updated?
When moving files from CS5 to CS4 - do you have any tips?
CS5 file must be exported to IDML which you can then open in CS4.
Don't expect the document to look the same as it does in CS5. Text will reflow and any new features will be lost or badly mangled.
In short, if you have to work with someone on CS5 you should upgrade.