I'm fairly new to quilting and doing a BOM with a LQS in my area. I should know better than to trust the cutting and measuring instructions by now. They haven't been right yet. But here goes...

If I want a 4 1/8 square block made from 4 quarter sqare triangles how big should the squares the QST's come from be? My instructions told me to cut 4 QST from 4 3/4 inch squares and sew them together. I'm supposed to get a 4 1/8 inch block from that? I only get a 4 inch block. What am I doing wrong?