I don’t mean to suggest that our universe is like the playing cards, only that we can understand how we would relate to a 4th dimension by understanding how the cards (a 2D reality) would relate to a 3rd dimension. If a seemingly 2D universe has a hidden 3rd dimension, then we can imagine different slices of that universe which occupy the same original 2D space, but are invisible to each other because of a slight displacement in the third dimension. By the same token, we live in a universe that appears to us as three dimensional, but if there is a 4th dimension, then we can imagine different “3D slices”, which occupy the same original 3D space, but are invisible to us because of a slight displacement in the 4th dimension.