What mechanics would a 4th race have to separate them from the others?
I think a cool feature for a 4th race would be "Permanent Death". Maybe slightly overtuned characters, but once they die - they are wiped clean. There's a niche community that really enjoyed permanent death features in Diablo 2, 3, Path of Exile, Everquest, and even World of Warcraft (Granted in the last two, there were no features to support it - it was on an honor system).
I think a permanent death race would add a "Wow, THERES A ______!" moment to any encounter.
Of course the race would be PvP flagged at all times like Demons.