C-x 4 is a prefix key for a variety of commands that switch to
a buffer in a different window—either another existing window, or a
new window created by splitting the selected window. See How
display-buffer works, for how Emacs picks or creates the window to use.
Select buffer bufname in another window
switch-to-buffer-other-window). See Creating and Selecting Buffers.
Display buffer bufname in some window, without trying to select
display-buffer). See Displaying a Buffer in a Window, for details
about how the window is chosen.
Visit file filename and select its buffer in another window
find-file-other-window). See Visiting Files.
Select a Dired buffer for directory directory in another window
dired-other-window). See Dired, the Directory Editor.
Start composing a mail message, similar to C-x m (see Sending Mail), but in another window (
Find the definition of an identifier, similar to M-.
(see Find Identifier References), but in another window
Visit file filename read-only, and select its buffer in another
find-file-read-only-other-window). See Visiting Files.
A more general prefix command affects the buffer displayed by the next command invoked immediately after this prefix command. It requests the buffer of the next command to be displayed in another window.
This general prefix command requests the buffer of the next command to be displayed in the same window.