Don't forget to cut your fabric for the snowball corner so the seam meets the top seam in the 9 patch. I cut squares the same size as my 9 patch squares (4.5 for you) and draw a diagonal line on it. Sew on the line and cut off the excess. If I were working with that size square, I'd sew a second line 1/2" inside the first. When you cut between the lines, you'll have a bonus HST.