If your finished border is 4" (4 1/2" unfinished) then your corner block needs to also be finished at 4." If you want to have 2 HSTs in each direction then they would be 2" finished (or 2 1/2" unfinished) for a total of 4 HSTs. This means that your starting square would need to be 2 7/8" (if you sew perfectly) or 3" (if you want to trim down).