Because this change underlies all the other quadrant views (you are only "he", when I am mainly I), it could be the quadrant in the middle or the big one beneath/encompassing the other ones.
Another problem I see in considering all quadrants equally. Who perceives the four quadrants and changes between them? Since consciousness is omnipresent, as Wilber states by himself elsewhere, the first-person view should be at the top, followed by the other three views being special aspects of the dynamic "I" (of the individual focus, the universal subjectivity). So the individual becomes a dynamic hierarchy culminating in the center of "I". It is from this perspective only, that "others" and collectives are possible in the last consequence.
The above hierarchy may be illustrated by this picture:
Meanwhile Ken Wilber has published a different explanation (consistent within its own framework) which presupposes – as most scientific concepts do – that we can't access another You completely. They say we can only approach it from its outside, from our own I. But how then do we know that there is anything to be "approached" at all? Strictly speaking, we can't. So, all these concepts lack deeper consistency.