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.
bufname in another window
switch-to-buffer-other-window). See Select Buffer.
bufname in some window, without trying to select
display-buffer). See Displaying Buffers, for details
about how the window is chosen.
filename and select its buffer in another window
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, similar to C-x m (see Sending Mail), but in another window (
Find the definition of an identifier, similar to M-.
(see Xref), but in another window
filename read-only, and select its buffer in another
find-file-read-only-other-window). See Visiting.