In this context, "four dimensions" refers to the spacetime continuum, made up of three spatial dimensions (length, width, height) and one time dimension. This concept was formalized in Albert Einstein's theory of relativity in the early 20th century and is called four-dimensional spacetime, or Minkowski space.
In modern physics, particularly in string theory and M-theory (branches of high-energy theoretical physics), models with extra dimensions are considered. String theory posits the existence of ten dimensions (9 spatial + 1 temporal), while M-theory posits eleven (10 spatial + 1 temporal).
These extra dimensions, beyond the familiar four, are considered "compactified" — curled up to extremely small sizes, making them unobservable in everyday life. Their size is thought to be on the order of the Planck length (roughly 10-35 meters).
Theoretical physics and cosmology also feature concepts of "hyperspace" and multidimensional universes, used to explain fundamental interactions and the structure of the universe. These ideas have also found their way into science fiction, where a "fifth dimension" is often depicted as a way of traveling between parallel worlds or overcoming the limits of three-dimensional space.