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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 section M.2 Visiting Files.
dired-other-window. See section AB. Dired, the Directory Editor.
mail-other-window; its same-window analogue is C-x m (see section Z. Sending Mail).
find-tag-other-window, the multiple-window variant of M-. (see section W.2 Tags Tables).
find-file-read-only-other-window. See section M.2 Visiting Files.