Both .ec4 to .ec3 files come from a software called Epson Print CD, albeit they might be used for a slight different purpose. But perhaps such ec4 to ec3 conversion is merely a way to import previously exported CD documents back to the tool and save these as projects again.
We did not find any specifics about such conversion or export, so if you have some particular issues, it would be perhaps best to reach out to Epson support.
Updated: September 11, 2023