Hm, in layman words, space-time is actually 4D (3+1 dimensions), and in strictly 'scientific' (physical) jargon, all calculations/integrations (especially in gravitational and high energy physics, and 'usual' quantum field theory; at least to my knowledge) are done over 4D - lagrangian density and consequently action, when 'calculating' with them, all contain something like:

where

*n* (number of overall dimensions; or

*D* in some notation) is usually 4.

www.scholarpedia.org

Or Einstein-Hilbert action explicitly, used in Einstein equation (general relativity):

That's not to say the above is correct description of Reality, just that people, when speaking about our everyday world, commonly consider spacetime to be 4D.