I'm clearly not the target demographic for this game, but I played it anyway and had a good time. This is a very sweet game and has pretty high production values, especially considering the fact it's free. I do have a few minor criticisms though. I'll order them from most objective to most subjective.
1) Typos
2) Package name issues
So I played Yuki's 4P on my Chromebook using its Linux container to run it. Unfortunately, the Linux terminal doesn't generally accept punctuation in file names. Because of the apostrophe in "Yuki's-4P", I had to rename some of the files in order to launch the game. Not a huge problem, but something to be aware of if the dev plans to release more games for Linux.
3) Whited out choices
As a fellow Ren'Py dev, I know why this is. This white text is, for some reason, the default when you select white as the GUI colour for your project. Thankfully, there's an easy fix. Just change the hex colour value in gui.rpy.
4) A floating cup?
5) "Lastname" and "Familyname"
This is purely my opinion, but I'm not a huge fan of Yuki's and Chinatsu's last names. I see what the dev was going for, but I think they ran too far with this joke, so it just appeared lazy. Personally, given the Japan-esque setting, I would have translated them into katakana and then to romaji. So Lastname would become "Rasutoname" and Familyname would become "Famiriname". That way the joke could be maintained, but it wouldn't be so painfully obvious.