Free EA release
You could include a mechanism inside the MT5 version that writes to the common folder .
Then you'd take the MT4 version (the freebie for those with the MT5 version) and create a special edition (not published on the marketplace) that would only operate if said "write" from the other utility has happened (and is correct , hash etc) within a time period recently .
You would compile that MT4 with the cloud compiler and would place it for download on a blog post .
That way you could link it in the MT5 overview (or the comments section of your product).