Concise Bible Dictionary:
This may perhaps be said to be the most perfect earthly shape of a plane (the “cube” being perfection for a solid). See “four” under NUMBERS. It was the shape of the brazen altar (Ex. 27:1; Ex. 38:1); the breastplate (Ex. 28:16; Ex. 39:9); and the altar of incense (Ex. 30:2; Ex. 37:25). Apparently it was the shape of the “panels” of the base of the molten sea in Solomon's temple (1 Kings 7:31); also of the court of the future temple (Ezek. 40:47); the altar of the same (Ezek. 43:16); the portion of the land offered as a holy oblation (Ezek. 48:20); for the sanctuary (Ezek. 45:2); and for the city (Ezek. 48:16).