In my case it completely didn't work. I downgraded the game to version 3.0 immediately after purchasing it, so I had no achievements in game. During my first playthrough, I died at the beginning, for which I received the Karma achievement, which should be obtained after completing the second episode. I completed the first episode with the Distrust ending, for which I did not receive any achievements. I finished the second episode with the Karma ending, for which I received the achievement earlier. After starting the first episode again, the game doesn't want to let me take an alternative route and doesn't show the ghost in the big window. Apparently version 3.0 is so buggy that it's a waste of time.

I answered this below, but the game installs on v4.0. The devs forgot to change the version number in game.

If I remember correctly, v3.0 creates a new folder where the save is stored. I think I have it listed in this guide somewhere. Switching between versions should just use their dedicated save locations.

As for the update, I'm not 100% sure. I have auto-updates turned off, but you might not even need to do that if you're manually downpatching.

Not to be even more annoying, but I still have a question:

If I do the downpatch, how do I prevent Steam from updating the game to 4.0? Do I just have to disable updates?

And another thing: you told me that if I do all the endings in version 3.0 I would have the complete gallery of endings. So far, so good. But how do I go back to 4.0 while keeping that? Do I just have to update? Because otherwise I would have to stay on version 3.0 forever.

My saves from version 4.0 don't make any difference, because they only have the endings. I haven't done the ghost list or anything extra yet, exactly because I haven't been able to complete the endings. My intention is: downgrade to 3.0, do all the endings, go back to 4.0 without losing that and complete the other lists, that is, 100% of the game.

Thank you for your tutorial.

I believe I recall seeing that the v4.0 still says v3.xx on the menu for whatever reason. You reminded me of that, and I should probably add that in here.

I already have v3.0 for some reason? Without patching, but I still can't get the ending. I got the candle, chrysanthemum, bear..everything, but it still doesn't trigger the cutscene

I have a full video at the top of the guide! It goes over the steps to not only downpatch, but also a quick walkthrough of the steps I took to get the ending.

Dear friend, thank you very much for the quick response. Yes, it made perfect sense and I expected it to be exactly what you said. It's just a shame that you have to resort to a downpatch, to make an ending, when you could just release a small update, correcting the trigger for the final cutscene. But anyway, let's wait.
Your tutorial is perfect and I completely understood it. I'm just afraid of doing something wrong and losing, especially the version 4.0 save, or it causing a bug when the version comes back.
Would you be able to post screenshots of the tutorial steps, or even record a short video with the steps? I think it would be even more perfect and would make it much easier to see what you have to do. But it's not a criticism, on the contrary. Just a suggestion, really. It was very well explained.
A hug from Brazil.

Glad I could help, dude!

Everything but Kkotnim Sweetheart will still be on your old (4.0) save when you go back, since you're doing that specific ending on a fresh (3.0) save. The only way to have your ending gallery completed is to either do every ending on the downpatch version, or wait for a potential update. Hope that makes sense.

Man, you're a hero. I'll use that guide, for my platinum completion.

If I downpatch the game and return after the achievement, every final that I make, will be there too?