The 4 string is a very different instrument to the 3 string, there is more scope in what you can do with it and so the approach to learning needs to reflect that. All of the courses here build skills, the 4 string course focuses a little more on building knowledge as well. The 4 string course is larger than the 3 string courses and covers areas such as scale/chord construction and musicianship. It represents a comprehensive approach to playing a more complex instrument and provides the essential knowledge you'll need for the workshops, ebooks and song lessons elsewhere on the site
The course itself builds the skills required to play fretted instruments with a flatpick and while you're at it learn some Rock classics from the Stones, Creedence, The Beatles and the like. Blues lovers will go for the Fingerpicking and the Slide Workshops that complete the best set of resources for 4 string guitarists anywhere in the known universe.
Because the main course looks at building skills in flatpicking and fretting and getting a strong repertoire of chords and scales the songs that help with that tend to be Rock tunes. When you learnslide and fingerstyle you'll want to be playing some blues and so that's what happens in the six 4 string workshops.