Cell address A4 in a formula means it is a Mixed cell reference

Relative cell reference.

Explanation:

Microsoft Excel is a software application or program designed and developed by Microsoft Inc., for analyzing and visualizing spreadsheet documents.

A spreadsheet can be defined as a file or document which comprises of cells in a tabulated format (rows and columns) typically used for formatting, arranging, analyzing, storing, calculating and sorting data on computer systems.

A relative cell reference can be defined as a cell whose reference is mainly relative to the location of the cell.

In Microsoft Excel, the cell reference is considered to be a relative reference by default.

Hence, the cell address A4 in a formula means it is a relative cell reference and when the formula is copied, the reference in the formula will change.

For example, if you refer to cell A4 from cell D4, it means you're pointing to the cell that is three (3) columns to the left of the same row i.e D minus A (D - 4).