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.
Select buffer bufname in another window
Display buffer bufname in some window, without trying to select
display-buffer). See Displaying Buffers, for details
about how the window is chosen.
Visit file 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 a tag in the current tags table, similar to M-.
(see Tags), but in another window (
Visit file filename read-only, and select its buffer in another
find-file-read-only-other-window). See Visiting.