The most unique thing about 4-dimensional space imho is that some simple manifolds like e.g R4 have uncountably many differential structures possible, in lower

The most unique thing about 4-dimensional space imho is that some simple manifolds like e.g R4 have uncountably many differential structures possible, in lower dimensions there is only one, in higher - finitelly many. When push comes to shove I'm not sure we have enough experimental physicists...