When Elena cuts from one corner to the opposite corner, she creates two right triangles. The diagonal cut is the hypotenuse of one of these triangles.
Using the Pythagorean theorem, we can find the length of the hypotenuse:
h^2 = 7^2 + 4^2
h^2 = 49 + 16
h^2 = 65
h = sqrt(65)
h ≈ 8.06
Therefore, the length of the diagonal cut that Elena made is approximately 8.1 inches (rounded to the nearest tenth).