Aloha zzzstephen,
As RoyalC130 says above, there is plenty of information, skills, and understanding of Qi, healing, and the emptiness to develope in Levels I and II to keep one busy for a long time.
Levels III and IV classes focus on building the energy of the "upper" dantian or third eye area, just as Level I builds the lower, and Level II builds the middle. No need to think one's missing out and jump into the "upper" classes. As Chunyi mentions, one needs to build a strong foundation (lower dantian), before building at the top or it's very unstable and can tople easily.
Chunyi recommends spending a good 3 months doing the Level II exercises and healing of others before moving into the Level III classwork. This allows most students the time to build their confindence, focus, and trust in the universe, to better absorb what is shared in Level III. The same goes for moving form Level III to IV.
Althought here is plenty of information given at these levels, a majority of it comes from the student's own questions and the time spent in meditation with Master Lin.
This is why we host a 6-day retreat with him every year - to provide the most opportunity for student's questions and personal healing and energy purifying with Master Lin. It reviews I & II on the first two days, gives plenty of opportunities for detecting and healing practice, questions, and goes into the Level III work as well as thing not normally taught in any of his classes.
Enjoy your home study courses.
Much Love, Shawn