that true. but theories are of different strengths in the scientific sense.
4D space-time is currently broadly accepted as being our current "truth".
I know, but the context of the thread is a "what if", and I'm assuming the what-if refers to the potential for additional spatial dimensions.
Even though I don't know of any strong evidence for additional dimensions (or for anything other than the large-scale flatness of 3D space, which limits the chance of additional spatial dimensions being as interesting as they could be), I think it's a valid hypothetical realm.