Squares

A square is both a rectangle and a rhombus: four right angles and four equal sides. Its diagonal cuts it into two right triangles whose legs are both a side of the square.

With side 4: ${d}^{2} = {4}^{2}+{4}^{2} = 32$, so $d = \sqrt{32} = 4\cdot \sqrt{2}$. Every square's diagonal is its side times √2. The ticks say BC = AB, so pick Marked equal sides for the second leg before Pythagorean theorem.

Triangle ABC is isosceles with a right angle at B, so its other two angles are 45° each: the diagonal cuts each corner in half.