If you are referring to MCTB 4th path which is considered arahatship by some people (I am not one of those people and do not consider it to be equivalent to the arahat of the pali canon suttas but more like an early fetter path), it's about all the phenomena, physical and mental, which includes the formation of the sense of self, being seen as equal, no better than, no higher status for one sensation over another. The whole playing field is leveled. No king of the hill. No stickiness or clinging element there. The mind becomes a teflon mind. No phenomena has anywhere to land or stick as it arises and passes away. Habitual tendencies still arise but do not stick. Thoughts arise but do not stick Mind states arise but the mind does not stick to them. There seems to be a choice now. You can choose to run with the phenomena or watch it arise within awareness and pass away within awareness. You do not "eradicate" the sense of self. It is seen for what it truly is and it's illusory higher status is taken away and it still arises and passes away like any other phenomena, just with no "stickiness" or clinging involved. For the first few days of this experience it was very surreal for me.
All the other previous paths seemed to occur after a "special" fruition. But this was different. It was more like a perception shift without a fruition. For myself, I had been agonizing over what to do to "get it done" for a couple of weeks. What the hell does one need to see? I had several pieces of advice by fellow yogis on what they did to get it done. One was seeing the "Non-sacred nature of ALL phenomena" which includes the formation of self. Another was to focus on where "suffering" was emanating from. Where was it all coming from. Also the fact the "I" had nothing to do with getting it done. The attachment to the sense of self was still there, ever so sublte. It was the 3rd path dilemma of what more was there to do??????
I sat there noting and noting the misery within. The mind shifted into high equanimity and dispersed. I stopped noting and the thought "Why are the sensations that make up the "I" being give more importance than all the rest of phenomena? Why is it still so prominent? Those sensations are no different than any other sensation. There is no difference. They are of equal status.
A fruition occurred. The mind rebooted. But it changed nothing. I kept questioning in the way mentioned.
As the mind kept at this way of investigating, the sensations and other phenomena propping up the of illusory "I" were seen as nothing more than sensations no different from any other sensation; the images and thoughts as no more important than the rest of the impermanent, not-self phenomena. And the experience within synched up with this form of questioning and investigation. Some sort of shift or blip or something hard to explain occurred. The mind just synched up. Blip. No fruition. The perception shifted to this way of seeing; Seeing all sensations including the ones clustering and being read together as self as all the same arising and passing away phenomena....permanently. No more stickiness. No more clinging to phenomena.
Then all sorts of thoughts arose. Doubt, curiosity, amazement. "Is this real? Did I just complete the circuit? Am I done? Is this real? I'm not sure." But these thoughts were being experienced differently now. They weren't sticking. They arose to pass away like cigarette smoke. So amazing. Negative sensations arose. But no rolling in them. Phenomena continued to arise, but no clinging. The formation of self continued to arise...but no clinging to it. That clinging element, whatever it was, is not there no more. And that was "insight disease". That clinging that caused so much misery. The second arrow is gone. Now, things just arise and there seems to be a choice, to run with it or not and just let it arise as an embryo and pass away as an embryo, without hatching and growing into a full blown flow of thoughts or emotions or mind state, rolling in attachment and misery. And the feeling of "doneness"......no more searching. A circuit completed! Now the rest of your life to be a human being....just no clinging anymore.
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My 2 cents
P.S. Edited to include the fact that I do not like the term arahat and would not say 4th path is the same as the 10 fetter model arhat seen in the pali canon. However, I believe that this long held traditional ideal of the arahat is possible and hope to one day achieve it. 4th path that is often talked about here and at KFD, is a perfect jumping board to go in the direction of 10 fettr model arahat.