Well in mind space, there will be many agents basing their measures on different things. For me, it is based my intuition about "caring about everyone equally," and looking at programs as infinite binary strings as you describe. That does not feel like a satisfactory answer to me, but it seems just as good as any answer I have seen to the question "Where does your utility function come from?"

I have read that post, and of course, I agree with your reasons to prefer 4.