It says that Riemann Curvature Tensor (term in left side of GR field equation), is a Rank-4 tensor formulated in 4 dimensions (space-time).

Therefore it is mathematically represented as 4-dimensional matrix (rows, columns, height, and "4th direction").

It is a mathematical MONSTER because it consists of 4 x 4 x 4 x 4 matrix elements (256 terms!).

Imagine performing matrix multiplication and other such operations with 256 terms arranged in four dimensions!!! Just keeping track of relative positions of elements in matrix would be a headache alone!