Assuming the "world" is at least multiversal leads to a couple of issues. One of which is Asriel's hypergoner feat, where "world" >= 2-B is used in a blog to justify Asriel actually erasing the timeline (since the world ending soon after should debunk this). One of the issues with this is that the save file still counts time after the timeline is destroyed. This should've been used as evidence for a downgrade instead of an upscale for the cosmology.
Another is sans dialogue, where the world ending is described as "sudden" or time-like. This is incompatible with the assumption that a temporal structure was erased. This also can't be used as evidence of a hypertimeline. Doing so results in a sudden destruction of a hypertemporal structure, requiring a hyper-hypertimeline, and the scaling becomes circular.
In addition, there are various statements that contradict a 4D "world." For example:
Flowey stating that Chara is no longer in the world. If the world were 4D this makes a simple death appear as EE.
Flowey claiming to be the prince of the world's future. This implies that the world is 3D as it is affected by time. Sans also makes a similar statement regarding the world's future but i don't wanna look for it lol.
If that isn't enough there's also Goner Kid's parallel worlds dialogue which establishes the world as a single universe.
This context implies that the "world" does not refer to the entire cosmology, but rather an H3-A structure. This doesnt contradict sans statements as "everything ends" is vague enough to mean "everything in our 3D universe." This should make Chara, who explicitly destroys the world, H3-A. Asriel can rewrite the timeline and possibly destroy it with hyper goner (if so then Frisk restores it immediately after), so he's L2-C.