if you want a 4 inch finished hst, sew 4 7/8 inch sqs diagonally to make two.
matching your seams of hst's is easier if you start with larger sqs and then trim. so sew 5 inch sqs diagonally and then cut and trim. but trim by laying your ruler on the hst's with the 45 degree line down the seam. that is a must. then trim, turn and trim again if necessary.
that way your seam lines will match up perfectly when sewing hst's together.