That's exactly the definition of a FINISHED block measurement - the size once it's sewn into the quilt.
So if, as in your op, you wanted/needed HST blocks that were FINISHED 4.5" that would mean 5" UNFINISHED.
As I said upthread, the formula is the FINISHED size plus 7/8" (or 1.25 for comfort).
You make the HST's, and then trim them down to the UNFINISHED size so you can sew them into the quilt.
Where they will be 4.5" FINISHED.