The story
From figure (“from the shape of the glyph "4"”) + 4 (“a figure akin to that shape”).
Etymology from Wiktionary, CC BY-SA 4.0.
figure 4
Today it means: A dynamic body position where a climber hooks one leg over the opposite arm (forming a "4" shape) in order to engage the core, maintain stability, and reach for distant holds, especially on overhanging terrain. Full definition →