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.
Select buffer bufname in another window. This runs
Visit file filename and select its buffer in another window. This
find-file-other-window. See Visiting.
Select a Dired buffer for directory directory in another window.
dired-other-window. See Dired.
Start composing a mail message in another window. This runs
mail-other-window, and its same-window version is C-x m
(see Sending Mail).
Find a tag in the current tag table in another window. This runs
find-tag-other-window, the multiple-window variant of M-.
If the variable
display-buffer-function is non-
nil, its value is
the function to call to handle
display-buffer. It receives two
arguments, the buffer and a flag that if non-
nil means that the
currently selected window is not acceptable. Commands such as
work using this function.