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 Window Choice, for how Emacs picks or creates the window to use.
C-x 4 b bufname RET
bufname in another window (
switch-to-buffer-other-window). See Select Buffer.
C-x 4 C-o bufname RET
bufname in some window, without trying to select it (
display-buffer). See Displaying Buffers, for details about how the window is chosen.
C-x 4 f filename RET
filename and select its buffer in another window (
find-file-other-window). See Visiting.
C-x 4 d directory RET
Select a Dired buffer for directory
directory in another window (
dired-other-window). See Dired.
C-x 4 m
Start composing a mail message, similar to
C-x m (see Sending Mail), but in another window (
C-x 4 .
Find the definition of an identifier, similar to
M-. (see Xref), but in another window (
C-x 4 r filename RET
filename read-only, and select its buffer in another window (
find-file-read-only-other-window). See Visiting.