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 main course is about building from scratch the skills you'll need 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. There's a little more theory in the 4 string course just because you've got all those extra options with the 4 strings. Those options also make the theory course and the advanced improvisation course a whole lot more appealing to 4 stringers.
So the main course sets you up with the basics but if you want to learn the dark arts of slide and fingerstyle guitar there are 6 workshops that take you through the skills and a stack of traditional blues to play. All up you've got the best set of resources for 4 string guitarists anywhere in the known universe.