Just curious, why do you ask?
Personally, I think it's tough to compare 4th path and a mental illness.
Mental illnesses or negative states listed above such as de-realization/ depersonalization or schizophrenia have more to do with being overcome or overwhelmed by mental phenomena rather than overcoming it, as is done in realization. At 4th path you see the non-reality, or non-solidity of thought, you see the empty nature of what previously seemed solid and fixed. Thus you have given up belief in the solidity of your conceptions, and you view the world in a manner less clouded by preconception.
From a normal unenlightened, not mentally ill, mind state or awareness, realization such as 4th path and mental illness are moving in opposite directions. For instance, if floating in space it your baseline state, realization would mean moving upwards, while mental illness would mean moving down.
I would be curious to hear about the cases you speak of in which realization caused a mental breakdown of sorts, but I doubt that this happens that much.
Sadalsuud Beta Aquarii:
I guess there must symptoms like:
1. no thoughts or actions appearing to be one's own
2. no belief in ability to control one's actions
3. inability to believe in own identity
4. inability to perceive differences between sensations relating to self and other
So here you are referring to what happens at 4th path, eh? Here is my take (as a post-4th pather) on what you have written, for the sake of clarification.
1. Kind-of. You see that you are not your thoughts, and this includes thoughts about actions, but you are still acting nonetheless. This goes with 2.; by all means you are still able to control your own actions and are fully aware of this, you are just not fixed on a solid conceptual image of yourself doing these actions. Actions are more pure, they are done, life is lived, in absence of a belief or fantasy about who you are and what you're doing. You just do things.
3. Inability to believe in one's identity is different from seeing that identity to be empty of solid, inherent reality. I am fully aware of being here right now, I'm just not having a massive conceptual train of though in my head about it. I'm just here.
4. Inability to perceive differences between self and other is not a result of attainment. Losing your self doesn't mean you lose anything, such as logical faculties of mind. This is something which people no doubt have a lot of misplaced fear about. Attainment, doing away with ego,
doesn't mean you lose anything. All your 'self' is is just fleeting thought which you have confusedly bought into as being real. What you lose is confusion. In fact you lose nothing, you just right your perception. Confusion is really the only negative thing we have, and that's what we lose on the path.
No sanity is lost!!!