The lone handler-mark (1) puts on a constant force (in this case; flick). The open-side mid (3) shouldn't worry about the dump throw unless it gains significant yardage. Deeps can't both get caught too deep; one can decide before the pull who should stay deepest.
Once the disc gets swung to the sideline, the mark essentially puts on a line-trap, with the break-side mid (2) pinching in and cutting off the dump. The break-side deep (4) also pinches in to cover the easy cross-court blade, with the open-side dropping deeper and becoming slightly more central.