I think you are saying you want to make a cornerstone that is made up of 4 half square triangles.
If you cut the border strips at 4 1/2 inches that means that after the seam allowances, your border is 4 inches finished.
That means your 4 half square triangles must finish to 2 inches each. No matter what the finished size is of the half square triangle, you need to add 7/8 of an inch to the finished size. I usually just add one inch to give me a little wiggle room. So cut your squares at 3 inches. I make the half square triangles and then trim them down to the correct size square. For you this is 2 1/2 inches.
Remember, each set of two squares will make two half square triangles.