Four Shingon emblems, aids to Yoga-possession by a Buddha or bodhisattva; they are 鉤, 索, 鏁, 鈴, a hook, a cord, a lock, and a bell; the hook for summoning, the cord for leading, the lock for firmly holding, and the bell for the resultant joy. Also, the four Veda śāstras.

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

（名數）鉤索鎖鈴之四攝菩薩種子也。秘藏記本曰：「真言行者能作此觀，以四明引入諸佛於己體。四明謂鉤索鎖鈴，鉤鉤召，索引入，鎖堅住，鈴歡喜。」【又】四種之吠陀論也。演密鈔二曰：「韋陀此云明，即是外道四明也。」毘奈耶雜事十六曰：「婆羅門子讀四明論。」最勝王經七曰：「大婆羅門四明法，幻化咒等悉皆通。」

丁福保《佛學大辭典》