I have an unstemmed kiddush cup with Hebrew inscriptions on the inside. I am told that it is from Rabbi Hashrash and refers to the 4 rivers of Eden. Do you know the story behind this?
Yes I do actually. It is a segulah, supposedly a surviving part of the book of refuot and segulot that was hidden by King Hezekiah. If made according the specifics brought down by the Rashash, and available online in the siddur of the 5th Rosh Yeshiva of Beit El Rav Raphael Yedida Abulafia(it runs on for several pages), the letters would be hand engraved with specific Kabbalistic intentions(or Kavvanot) and prayers while being engraved.
Today there are two versions of this cup commercially available, one is with hand engraved letters, and one is factory produced with laser engraved letters. According to Rav Benayahu Shmueli, in his sefer Pidyon Nefesh p. 263(Second Edition published 2011/5771), the latter is inferior, but still a valid segulah. As well he brings there(p. 264) a prayer to be said before usage as well as various other usages for the names and how that can be accomplished.
Quoting directly from the text: