C-x 4 is a prefix key for commands that select another window (splitting the window if there is only one) and select a buffer in that window. Different C-x 4 commands have different ways of finding the buffer to select.
find-file-other-window. See Visiting.
dired-other-window. See Dired.
mail-other-window; its same-window analogue is C-x m (see Sending Mail).
find-tag-other-window, the multiple-window variant of M-. (see Tags).
find-file-read-only-other-window. See Visiting.
By default, these commands split the window vertically when there is
only one. You can customize the variables
split-width-threshold to split the window horizontally