You can roll back your Spine project to v3.8 from v4, but as you wrote, warning dialogue will pop up when you export your project as v3.8. It depends on your project whether the roll back would lose lots of information or not. If you are using a lot of features that are not compatible between 3.8 and 4 like separating X/Y axis, it may be hard to roll back completely. If you haven't done a lot of work with v4 yet, you can safely ignore the warning.
I had written about how to rollback in the following topic, so it will help you if you'd like to do that :
viewtopic.php?f=9&t=16597
