Marcin Borkowski (mbork) has a very nice review of the【Ctrl+x 4】key bindings. Most of us learned—or at least read about—those when we started with Emacs but probably forgot about them because they were too esoteric for us then. That’s where his post is useful: it’s an excellent reminder of those key bindings and the functionality they provide.
For example, did you know, or remember, that 【Ctrl+x 4 c】 provides a clone of the current buffer in another window? Or even that 【Ctrl+x 4 f】 opens a new file in the other window? There are others as well so unless you’re up to speed with the 【Ctrl+x 4】 key bindings, head on over to mbork’s post and take a look.
UPDATE: Borkowski’s server appears to be having problems. Neither the above link nor those of previous posts about him are working.
UPDATE 2: MaikuMori comments that you can see the post here.
UPDATE 3: Looks like Borkowski’s site is back up.