There is also a Category V, which requires twice the study hours of Category IV (2200 hours) [1].
Last I heard, Category V was an unofficial level, and Korean was the only language in the category. It seems they made it official and shifted the languages around (interestingly, German is only language in Category II, now. And Japanese is considered more difficult than Korean.)
https://www.state.gov/m/fsi/sls/orgoverview/languages/
They break languages (note: that they teach) into four categories:
* Category I: Languages closely related to English.
* Category II: Languages that take a little longer to master than Category I languages.
* Category III: Languages with significant linguistic and/or cultural differences from English.
* Category IV: Languages which are exceptionally difficult for native English speakers.