Not all airports can be reached in 4 hops from all others - see this Travel StackExchange question for examples. Examples are something like Adak, Alaska (which only has service to Anchorage) to Chimbu, Papua New Guinea (which only has service to Port Moresby, the capital of PNG); Anchorage to Port Moresby requires three hops, something like Anchorage - LAX - Brisbane - Port Moresby). I'm not sure if that explains the length differences, though.