Unfortunately, it is not possible to integrate multiple versions of the spine-unity runtime within a single Unity project, so the only way is to match all of your Spine projects to 3.8 or 4.0.
Regarding the issue of images being missing when importing JSON files, if the images path is corrected to an appropriate one after import, the images will be displayed properly. Did you try it because you want to downgrade a Spine project created in 4.0 to 3.8? If you would like to know the detailed procedure, please tell me what you want to achieve by importing JSON files so that I can explain it.
