Originally Posted by MTS

Did each of those blocks measure 4.5" BEFORE you put them together?

My guess is it's the HST block that was a bit small.

Try the method again, but this time start with 5.25" squares.
Make the diagonal line, sew on each side, cut and press.
NOW trim it down to 4.5" square.

I always use this "cut big and trim down" method. It results in a perfect square each and every time.
4-7/8" is exact...if you're off the slightest bit while cutting or sewing, you're never going to get the correct UNFINISHED size.