So I'm gonna say it here real fast, this is gonna be short since I want to show progress, but then I forgot about everything, so I got fast to work!
Let's do this!
The Void
Wait isn't that Act 3?
Well no, It's actually where Sans dies and you reached the absolute.
Here are some screenshots!
( Note that I don't have much to put besides this so sorry. )

I changed the sprite of Chara to fit it more and I even made a new Frisk Sprite!
Now one to the next one!

Spooky, amirite..?
BOOOO!
That's what ghosts says, so spooookyyyyy.
Anyway since I don't have much I'll just say that:
There is no ACT 4.
There will never be an ACT 4, and if you think there is, or if it's official. Then this would be ACT 4.
Now for the last screenshot.

[999999999999999999]
So yeah that's all for the Halloween dev-log!
I'm sorry that it was so short, but I did have to show the chara encounter.
Well, see you guys next time!

-UTTSans