正方形 = square - a four sided shape with sides of equal length 四角 = rectangle, square; for further explanation, the word 角 means "corner." 四角 literally means "four corners," but the meaning of those words says nothing about the length of the sides of such an object. So 四角 is simply any shape with four corners. I had to look up the other two for 円い and 丸い, but I found the answer: 円い describes more 2D characteristics, or anything that is flat, but has a circular shape. For example: この皿が円い。 "This plate is round." Also 円 means circle as well, so you can use this Kanji to describe that shape. 丸い describes more 3D characteristics, or anything that is spherical, such as a ball. For example: 球・ベースボール・卵、全ては丸い。 "balls, baseballs and eggs are all round." All of these are spherical objects, so they can be described using 丸い. I hope this clears up everything. If you need more assistance, let me know. Best of luck in your studies!