4D usually refers to 4 spatial dimensions.It looks like two spaces are merged on-top of each other.
I'm not sure I understand what you're thinking but there's definitely no merging. It's just one 4D space that is limited to a 3D slice when displayed.
just watch the 4D minor video. there is a 1 minute 2D render of 3D space analogy that is easy to understand. then just extrapolate the idea to our 3D render of the 4D space we can't understand.That's why I was asking if two 3D slices were laid on-top of each other like that 2D cross-section.