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. 27:11And thou shalt make an altar of shittim wood, five cubits long, and five cubits broad; the altar shall be foursquare: and the height thereof shall be three cubits. (Exodus 27:1); Ex. 38:11And he made the altar of burnt offering of shittim wood: five cubits was the length thereof, and five cubits the breadth thereof; it was foursquare; and three cubits the height thereof. (Exodus 38:1)); the breastplate (Ex. 28:1616Foursquare it shall be being doubled; a span shall be the length thereof, and a span shall be the breadth thereof. (Exodus 28:16); Ex. 39:99It was foursquare; they made the breastplate double: a span was the length thereof, and a span the breadth thereof, being doubled. (Exodus 39:9)); and the altar of incense (Ex. 30:22A cubit shall be the length thereof, and a cubit the breadth thereof; foursquare shall it be: and two cubits shall be the height thereof: the horns thereof shall be of the same. (Exodus 30:2); Ex. 37:2525And he made the incense altar of shittim wood: the length of it was a cubit, and the breadth of it a cubit; it was foursquare; and two cubits was the height of it; the horns thereof were of the same. (Exodus 37:25)). Apparently it was the shape of the “panels” of the base of the molten sea in Solomon's temple (1 Kings 7:3131And the mouth of it within the chapiter and above was a cubit: but the mouth thereof was round after the work of the base, a cubit and an half: and also upon the mouth of it were gravings with their borders, foursquare, not round. (1 Kings 7:31)); also of the court of the future temple (Ezek. 40:4747So he measured the court, an hundred cubits long, and an hundred cubits broad, foursquare; and the altar that was before the house. (Ezekiel 40:47)); the altar of the same (Ezek. 43:1616And the altar shall be twelve cubits long, twelve broad, square in the four squares thereof. (Ezekiel 43:16)); the portion of the land offered as a holy oblation (Ezek. 48:2020All the oblation shall be five and twenty thousand by five and twenty thousand: ye shall offer the holy oblation foursquare, with the possession of the city. (Ezekiel 48:20)); for the sanctuary (Ezek. 45:22Of this there shall be for the sanctuary five hundred in length, with five hundred in breadth, square round about; and fifty cubits round about for the suburbs thereof. (Ezekiel 45:2)); and for the city (Ezek. 48:1616And these shall be the measures thereof; the north side four thousand and five hundred, and the south side four thousand and five hundred, and on the east side four thousand and five hundred, and the west side four thousand and five hundred. (Ezekiel 48:16)).