This is the fourth in a series of videos about the merge sort. It includes a description of some pseudocode which combines into a single recursive function a helper program for splitting a list, and a helper program for merging a pair of ordered lists. This video describes how successive recursive calls process successive portions of an original list, along with the role of the call stack. The videos that follow include a description of an implementation of a merge sort in Visual Basic.NET and a discussion of the complexity of the merge sort algorithm using Big O notation.