Nice! I can't claim to be an expert on 4-dimensional beings in another universe, but here are my thoughts.
As I see it, our vision provides two 2-dimensional pictures of our world which, roughly speaking, are made up of the nearest opaque points in each direction (Note that the space of directions from a point is like the surface of a sphere, and a small part of this is quite like a piece of the 2-dimensional plane).
If a 4-dimensional being existed in a universe with 4 spatial dimensions, it could feasibly have a view of the world which looked 3-dimensional rather than 2-dimensional. If our world was a section of their world, they could potentially see us as a detailed 3 dimensional thing, like the set of sections from some types of hospital scanner, only more detailed.
I suppose the argument for things in our universe not being 3D sections of 4D things, is that things (such as us) don't have mysterious behaviour that is unexplained by the 3D part.
This may be off-topic even for this group. But my hope is that I could find some help with this problem from people who seem to understand much more difficult problems.
My question is kind of a Flatland kind of question, but also involves time.
In the usual examples, if a 3D object sticks their finger in a 2D world, the 2D beings see a series of 2D images. What the 2D being views as a 2D image appear out of nowhere, and change through time, the 3D being views as a single unchanged 3D object that moved through space. The usual examples are of simple geometric shapes.
My opinion is that those were just for the sake of simplicity. But that any 2D shape possible to be viewed by a 2D being could be a slice or set of intersecting points between the particular 2D world and a 3D being. So that the types of shapes possible would not be limited to simple shapes.
Similarly, my view is that if a being that extends in n dimensions in space, where n is greater than 3, intersected at some points with our world, that we would be able to view a series of 3D slices that could be of any imaginable 3D shape, including that of a human, and that what we viewed as a single 3D person changing throughout time, could be a single unchanged slice of that n-dimensional being.
The debate eventually came to the subject of expressing this mathematically. I am trying to picture a way to describe this mathematically, providing it is possible to describe a section of 3D history of someone mathematically in terms of a higher dimensional slice of a higher dimensional being that is simply moving through space at an axis other than the ones we are familiar with normally.
My initial thoughts are to break down the contour's of a human into smaller pieces individually describable as simpler shapes, and then to write out ordered pairs of coordinates, showing how the 3D shape slices were subsets of the n-dimensional slice.
If you did not start with a simple geometric shape, but instead started with a life form that happened to have a human like shape, and it extended in space with that sort of shape in more directions than a real human, if they intersected with our world, would the 3D intersection look like a human or something else?