According to this website, you should add 7/8" to the finished size. Finished size is after it's pieced, so I think yours would be 4 7/8".
http://www.trishstuart.com/pdf_files/math_hst_qst.pdf
This second website illustrates more, but it doesn't seem to be as straight forward.
http://www.trishstuart.com/pdf_files/HSTmath.pdf
I hope this helps!