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. Window Choice, for how Emacs picks or creates the window to use.

C-x 4 b bufname RET

Select buffer bufname in another window (switch-to-buffer-other-window). Select Buffer.

C-x 4 C-o bufname RET

Display buffer bufname in some window, without trying to select it (display-buffer). Displaying Buffers, for details about how the window is chosen.

C-x 4 f filename RET

Visit file filename and select its buffer in another window (find-file-other-window). Visiting.

C-x 4 d directory RET

Select a Dired buffer for directory directory in another window (dired-other-window). Dired. @c Don't index C-x 4 m and compose-mail-other-window @c here, they are indexed in sending.texi, in the "Sending Mail" node.

C-x 4 m

Start composing a mail message, similar to C-x m (Sending Mail), but in another window (compose-mail-other-window).

C-x 4 .

Find the definition of an identifier, similar to M-. (Xref), but in another window (xref-find-definitions-other-window).

C-x 4 r filename RET

Visit file filename read-only, and select its buffer in another window (find-file-read-only-other-window). Visiting.

C-x 4 4

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.

C-x 4 1

This general prefix command requests the buffer of the next command to be displayed in the same window.