When you say 4.25" square cut into 4 (triangles), that means that you have 2 bias edges & it is a QST instead of a HST. That means you have 2 bias edges instead of 1 & it's easier to stretch out of shape.
I'd probably go with a HST cut from a 3" square instead of doing QST's. It will make it easier to keep the fabric from stretching out of shape. Other than that, your measurements are correct.