In this context, The 4th dimension DOES NOT refer to time, but a true fourth spacial dimension.
Consider a 2d plane with only an x and a y axis and a living creature whose perspective is locked to it. That creature would be fully unaware of anything outside of their tiny slice of reality. Now imagine a 3d person from our reality with x, y, and z axes looking at that plane from the side. They would be able to see aspects of that plane that are hidden even to the creature on it. for example, imagine a closed, hollow box existing only on the 2d plane. The 3d being would be able to clearly see what is inside the box.
To a being in a higher plane, a fourth dimension with w,x,y,and z,axes,our 3d existence is just as flat as the 2d plane discussed earlier.
Mysteria as a location, has at minimum, 4 spacial dimensions, meaning that all locations exist simultaniously even if they are located on separate facets.
What even is a Facet anyway?
Using the example of the 2d creature from earlier, imagine a varied 3d space full of different biomes. Then imagine that the 2d plane moves and rotates into a different location on that 3d area. the "Facet" refers to the exact "angle" and perspective that the 2d creature is able to see. It is called a facet because just as with a faceted gem, what you see depends on your perspective of it. There are theorised to be a near infinite number of different facets that make up Mysteria.
Moving between Facets:
Usually, when one is in mysteria, the plane they are in may shift at any time, bringing them to a new facet. This usually occurs in one of two ways, When the plane shifts slowly, it is called Blurring, this is when the surroundings morph and transform slowly from one Facet to another. The second is called a Chaos Storm. Think of a Chaos Storm as Blurring, but really really fast. like up to several times a second in extreme examples.
Some items, such as the Anchor Charm, (See Charms) can stop an individual from unintentionally blurring between Facets using a technology not fully understood.