Square or rectangular in shape; having four corners (adjective).
四角い箱にプレゼントを入れた。
I put the present in a square box.
あのテーブルは四角いので、端に物を置きやすい。
That table is square, so it's easy to put things on the edges.
彼の手紙は四角い字で丁寧に書かれていた。
His handwriting in the letter was neat and blocky (square-shaped characters).