Definition of figure 4:
(n) :
(climbing) A technical climbing technique used in ice climbing to gain a respite and lessen slack and distance from the undersurface/ceiling which one is climbing underneath, which appears like a number "4" in shape, with one arm anchoring the climber to the surface while hanging upside-down, and one leg wrapped around that arm, forming a figurative "4" in shape.
(n) :
(combat sports) Alternative form of figure four. [(combat sports) A grappling position where a fighter grabs an opponent's limb and then holds on to his or her own wrist with the other hand. Used in various joint locks.]