The rule of thumb is to add 7/8" to the finished size.
So to make two 4-1/2" unfinished HSTs that finish at 4" each ... you would cut two 4-7/8" squares, and sew together on the diagonal.
My preference is to add a full inch, so that I have a little extra fabric and can trim them to a perfectly squared HST. Makes for much nicer finishes to the projects.