Many knowledgeable pundits, especially those from the Antipodean region, consider the two man 12-foot skiff the most difficult dinghy to sail. I, never having seen one, will concur readily after seeing this video.
4th rig refers to the smallest racing rig for the heaviest air. The 12-foot skiff allows different rigs to be switched in and out depending on the wind strength.
(Those of the older generation might want to turn down the annoying, droning techno-beat sound track.)