I'm not sure I agree you can't visualize a 3d object. You can't draw one, I'll give you that, but we spend all our time in 3d space. Saying the brain's representation of that is 2D seems dubious.
Not the brain, the eye's representation is 2D. The brain's can be 3D, despite the brain not receiving any 3D input, so I don't see why the brain can't represent 4D as well.