Four-dimensional space, also called Euclidean four-dimensional space, is a standard Euclidean space. It is a mathematical concept, which can be extended to N dimension. The fourth dimension of four-dimensional space refers to the spatial dimension with the same properties as X, Y and Z.
In physics and mathematics, a sequence of n numbers can be understood as a position in an n-dimensional space. When n=4, the set of all these positions is called four-dimensional space. Four-dimensional space is different from the three-dimensional space of human life because there is one more dimension.
Through the evolution of one-dimensional, two-dimensional and three-dimensional space, people have put forward some conjectures about four-dimensional space. Although these conjectures can't be proved to be correct yet, many scientific theories begin with conjectures. Nowadays, scientific theories are generally based on summarizing the laws of phenomena, but there is not enough accurate and clear understanding of the phenomena in four-dimensional space, or we did not think that this phenomenon was caused by four-dimensional space after seeing it.
Of course, the topological space of points, lines and surfaces can also be defined as the fourth dimension, the fifth dimension, the sixth dimension or even the nth dimension. This is easy to realize in mathematical formula reasoning, but it is difficult to correspond and imagine in reality.
However, when people refer to "four-dimensional space", they are often confused with "four-dimensional space-time" mentioned by Einstein in his theory of relativity (called "Minkowski space").