As far as I understand, polymorphic relations are not ready to use yet as of version 4.0.2. They were not yet added to the v4 documentation and seem not to work smoothly so far.
The database layer appears quite ready and is already used by the file type in the upload plugin. However, currently the default controller strips off the relations during sanitizing the entities (at least for the morphOne and morphMany relation types).
@Mohammad_Ashenab, did you find any working solutions?
@DMehaffy, are contributions welcome regarding these issues or is this still work in progress by the strapi team?