Font Awesome 4.0 is a complete rewrite of Font Awesome from the ground up. One of the major new features is that icons are now namespaced. In addition to namespacing the icons utilize a new naming convention based on consistency and predictability. Unfortunately, this new naming convention is a vast departure from Font Awesome 3.2.1.
Is there an upgrade guide from from Font Awesome 3.2.1 to Font Awesome 4.0?
After a fair amount of Googling came across this Stack Overflow answer which in addition to other documentation questions about Font Awesome 3.2.1 links to a Font Awesome upgrade guide from 3.2.1 to 4.0.0.