I think I know how to get to the fourth ending. When you complete the first ending you get a set of blue numbers, I got 74, when you complete the second ending, you get a set of pink numbers, 69 for me, and lastly, the high lust and awareness ending, the third, get a grey number, I got 7 for this.
I assume these correspond with the Awareness Points, Lust Points, and Action Limit. If this is correct, then you'll need to match the numbers you got with the Points and Action Limit, which will require a lot of math. It'll be like inputting a sequence of numbers, get it right and you get the secret! if not, then you have to start all over from the beginning.
One more thing before I go, I believe 8GSUiSLvYz3ONkc5, the guy below my comment is most likely a scammer, don't click the link.