I think hop 1 is the "target" of the application, e.g. Carter Page. Then hop 2 is anyone that target talks with directly. Hop 3 is anyone that hop 1 or 2 directly communicates with, and hop 4 is anyone that 1,2, or 3 communicates with. So you can be pretty far removed from the person for whom you got the warrant and still spy on them (e.g. Trump).