3D being a cube or all the corners of 2 parallel planes connecting-A 3D being would be one who’s understanding contains depth. A dog would be 2D-3D, mostly 2D with a little bit of depth.
4D is 2 parallel cubes with lines connecting each of the cubes corners connecting to the other cubes corresponding corner- As the fourth dimension is time and since time reveals change, a 4D being I believe would be one who’s understanding is constantly changing, a being who is never content with his current understanding and is relentlessly trying to poke holes in his views in order to find better ones. Humans are 3D-4D ,mostly 3D but transitioning to 4D. What holds most people back who are trying to become 4D is the constraints of the status quo or the reigning beliefs of the time period in which they live. A fully 4D being would have no such constraints of the mind that could hold them back.