When you sew the four patch, have the raw edges of the seam on the top facing away from your body and the raw edges of the underneath seam facing your body. The machine pushes the top against the bottom one and the seams butt nicely making a perfect corner. Are you pressing all seams before they are crossed with another? My mother used to say'You sew with your iron, talking about apparel, but I think it applies to quilting, too.