I am still using Font Awesome 3 in my app and it seems that the docs are now v4 only? And version 4 changed the names of the classes for icons now so I'm not entirely sure which to use? For example, fa-check-square in v4 isn't icon-check-square in v3 as you'd expect, but icon-check-sign.
I have the same problem. I don't find how to access to information about the previous versions into the official page. In this version It will be a big problem because the class names was changed. I hope that the FontAwesome team solve it soon.
http://fortawesome.github.io/Font-Awesome/3.2.1/ answered in https://stackoverflow.com/a/19569853/1203628