Prime backwards compatibility - solved? Or at least hacked
Given how little versions changed, it has frustrated us all that we cannot save in older versions. I suspect this is largely a marketing ploy to force upgrades and new purchases. And, probably controlled by a simple version check. So after a little thinking, digging and experimenting, I have been able to open 5.0 files in 4.0. I haven't played with it much and I'm sure it's limited - but it might help someone someday.
With the new version file
a.) Change the extension to .zip
b.) Unzip to a folder
c.) Delete app.xml in docProps folder
With the older version
i.) Save any file
ii.) Change the extension to .zip
iii.) Unzip to a folder
iv.) Copy the app.xlm in the docProps folder
v.) Paste this 'old' app.xlm into the folder where you deleted the last one. You should just be able to re-use this file anytime you want to do this.
Go back to the 'new' version folder:
Re-Zip all folders and files that had been unzipped.
Change extension from .zip to .mcdx
Open in old version
In short, replace the app.xlm file with a version from the older mathcad
(Use at your own risk, no guarantees nothing else changed. Obviously won't bring new features across)