The four wheels or circles: (1) 大地四輪 the four on which the earth rests, wind (or air), water, metal, and space. (2) Four images with wheels, yellow associated with metal or gold, white with water, red with fire, and black with wind. (3) The four dhyāni-buddhas, 金剛輪 Aksobhya; 寳輪 Ratnasaṃbhava; 法輪 Amitābha; 羯磨輪 Amoghasiddhi. (4) Also the four metals, gold, silver, copper, iron, of the cakravartin kings.

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

四輪===（名數）四種曼荼羅也。曼荼羅者輪圓具足之義，故名四輪。【參見: 曼荼羅】第一則附錄。【又】有種種之四輪如大地四輪項。

丁福保《佛學大辭典》