I believe this is about 4 space-like dimensions, not spacetime. Imagine perceiving a 2d image from within the plane: you'd only see width, not height, and lines would be hidden behind other lines. Only if you're in 3d, you can look at the plane from atop and see everything. Similar for looking at a 3d scene from 4 dimensions.
Or is that described elsewhere? I've not read the book (one more for the quarantine list!)