Four appears to be an very bright star that glows blue and white. It is unlike all the other verses in the One Chain because it looks extremely bright instead of completely black.
However, Four looks just as dark as all the other verses in the One Chain when viewed from inside.
Four contains exactly 4 Threes inside of it, thus containing 4! (24) Ones. Between each Three is a ridiculously sticky and slightly pink substance. This substance is very hard to traverse, and stops all the Threes from bouncing everywhere.
However, if a Three glitches through reality, it can avoid the substance and still move freely, since it is in a different existence state.