Yes I missed out on a few secrets just so that I could have a TOO TOO joke in there, but if you are mad about that then here, some more secrets regarding chapter 4

23. When playing the piano there are several unique songs that will get a reaction, such as Rouxls Kaards theme, Megalovania, The Legend, Don't Forget, Flowey's theme, and many others (there are several videos covering it, I would recommend looking them up)
24. In the second dark world section, if you got the original starwalker in the first chapter then you will encounter a statue of it when in the room with many statues
25. Apparently there is a secret sound that plays when backtracking after Susie breaks the piece of the prophecy, though it is too quiet to hear, but when turning up the volume it is the same sound as when Susie attempts to play the guitar for Noelle.
26. There is a telescope in the room with 2 pianos, which will allow you to view any of the characters in the party, that being Susie, Ralsei or Kris, which Susie and Ralsei have special interactions for
27. When in castle town you can put several different characters at the same table to get unique dialogue, especially prominent with Elnina and Lanino, along with Miss Mizzle

And if there are any others that I missed I'm sure that you will all yell at me in the comments about those, so I'm not to worried about not having everything here, anyways hope you are all having a good day!

Ok so, I think Asgore’s Prophecy more refers to the definition of Asylum in terms of Seeking Shelter/ Safety from another source, as in Asgore is trapped by the Support of Carol, as she’s keeping him afloat, but he can’t stray from what she wants now.

at the end of the stained-glass hallway where you can get the egg, you can find the sacred moss

Man, nobody really talks about that +1 life point for Kris

the 'mike' fight was clearly supposed to troll streamers and i LOVE IT

5 dollars for 2 people at a diner is crazy

9:16 fun fact if u look into the files there is a guitar sound that plays but the volume is so low it can barely be heard, found by halfbreadchaos support le man

If you talk to Noelle, she opens the gate. Now go get the 5 dollars at Kris's house and go eat at the diner. Noelle is waiting the whole time and comments on it. Pretty funny

5:30 If you have done the weird route, Noelle also appears as an option to think about, where she says "...thank you."

4:46 technically Deltarune IS the older sibling as Undertale was made as a test run by tricky tony to see if he could actually make Deltarune happen.

9:25 there is also a VERY quiet sound of a guitar playing here, which might point to it being Dess's hand.

4:34 - If you follow the old man and he asks you where do you want to go next, if you say further left, a path will appear and you will be able to increase Kris's maximum health by 1 permanently. This is inaccessible if you pick any other dialouge option so keep that in mind
6:40 - During said fight, you are not allowed to use items making healing almost impossible... Except it doesn't. In this fight, you don't need to attack with anything at all however normal attacks give the most TP out of all interactions so do that a lot. The old man reduces his damage depending on your health amount making the fight significantly easier. And about healing, the only healing that you can use is Susie's OK Heal that initially heals 30HP. HOWEVER after each use, the healing gives 2 more health and uses less TP capping at 30->58 HP healed and 85->82 TP usage. Healing is the best and only good way of using TP due to Rude Buster not making any impact on the boss and being capable of striking back the player which can result in loss

Edit: My bad. Susie's heal TP usage can go down to 80TP. I confirmed it after fighting the boss again but I couldn't get it to heal more than 58 health.

I just realized "Mike" sounds like "Mic" I feel so dumb lmaooo

1:03 Noelle has a few different dialogue possibilities here. One I know is if you open the gate, then go to the Dark World for Ralsei's tea party, Susie talks about how they went to one and Noelle is confused. I think there's another special dialogue but I don't remember what it is.

Hi! i don't know if anyone's found this yet but i wanted to share it.

Yeah so from what I could tell there's a really missable area in Susie's dark world section that i believe has the same kind of secret face like the spooky face in chapter 2's basement after you ride the teacups back up. You need to find the area where you have to touch the moon piece to darken the area to progress and there's a hidden upper path that leads to the left into a very dark room with a yellow bell platform that immediately disappears when you step on it and illuminates the entire room for a brief moment so you can find a hidden chest. I was sceptical of the room as the music in general cuts out when you enter it and the path to the room entirely disappeared after i exited the area, so i reloaded my save to try to find out what was going on. Turns out if you run to the far side of the room where you entered and follow the yellow light, you can actually see what looks kind of like the face from the basement of chapter 2. You can only activate the bell once so it's a blink and you miss it moment but i speculate that since the room is so dark like the basement it's an opportunity to see things like the spooky face as you pass through these quiet areas. If you need it i can provide exactly where i found it, but i think it's a really obscure and cool easter egg.

If you keep telling the pumpkin jokes eventually it makes you tell a story even if you select joke and then it leaves after the iconic line.

You can actually go right inside the Asgore lore room to find additional lore about what the angel looks like.
Also, if you gave Ralsei the gift in chapter 2, in the carnival room, and then in chapter 4 go upstairs in the castle town. Head over to the left after having a tea party and interact with the barrier. Susie will insist on knowing what Ralsei's hiding. She will run through the barrier into his room. More dialogue and lore about Ralsei. If you gave the present to him, there will be a plush of him in the corner.
One more thing. Im not sure how it happened but at one of the hometown segments, I ran into the "normal npc" where you meet Onionsan.

Fun fact: if you turn on a Santa Guy before leaving the holiday house after the Carol cutscene, Carol will say bonus dialogue which is funny af

maybe Asgore got his shard the same way Kris did, by fighting the knight? as in, the thing that got Asgore fired was him going into a dark world to find Dess, he fought something, but couldnt describe what he experienced in a way others could understand, so afterward people thought he was being crazy or traumatized by the "real" thing that happened, which then made everyone think "Asgore is just lying and making things up, that's very sus, we don't want you to be chief of police anymore". he's said stuff about wanting people to believe him this time. maybe part of why he's working with Carol is so he'll be able to show everyone what really happened, by recreating what happened in another dark world, or just show that dark worlds exist. so that he can get back what he lost.
OR he just wants to create a perfect dark world of his own where it's 10 years ago and the family is still together and all is hunky dory.

9:12 to add to this fact: someone datamined and found that after finishing this dialogue, a sound plays too low to hear normally, but if you make it louder, it's the noise December's guitar makes when you knock into it or Susie plays it.