Spoiler for level 4's and 5's:
That sounds like a pretty sound theory Guido, maybe we could add that the soul remnant of a level 4 has been reduced to the state of a new born child, albeit a tortured, lonely child with no sense of self in a dark room with the door locked.
And to make it even more creepy we could presume that the soul remnant is gibbering madly in an incomprehensible voice that screeches for something (we don't know what its screeching for, cause we can't understand it) probably salvation.
Oh and for level 5's, if they have no soul, could the very void of their existence sap/suck out the souls of anyone in close contact with them? That would be quite the monster: Incredibly strong, agile, intelligent, and oh yeah, its also sapping the very essence of who you are as you fight it.
BTW, I am new to this particular section of the forum, so hi all.