Four poisonous snakes (in a basket), e. g. the four elements, earth, water, fire, and air, of which a man is formed.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

四毒蛇===（譬喻）以四毒蛇喻地水火風之四大也。涅槃經一曰：「自觀己身如四毒蛇。」同二十三曰：「有王以四毒蛇盛之一篋，令人贍養。」最勝王經五曰：「如四毒蛇居一篋，此四大蛇性各異。（中略）地水二蛇多沈下，風火二蛇性輕舉，由此乖異眾病生。」

丁福保《佛學大辭典》