Good question, and I think different curricula might approach this differently. Some might choose to introduce some of these Grade 4 concepts in an informal way in Grade 3. But I would point out that the classification into squares, rhombuses, and rectangles really only requires an ability to recognize angles as square angles or not, and an ability to detect when pairs of sides, or all four sides, are equal in length. This could be the right way to approach this with 3rd graders.