Welcome to the quilting board!
Actually, you've almost answered your own question. If you know the width of your quilt -- 90" -- and the size of your finished blocks -- 3.5" (you will lose 1/2" with your 1/4" seam allowance on opposite sides of each block) -- the you can just divide 90" by 3.5" and come up with 25.71 blocks, OR 25 blocks for convenience.
You will need 25 blocks across and 25 blocks down (same measurement both directions because the quilt is square) so you will multiply 25 x 25 for a total of 625 4" squares/blocks you will need.
This is a wonderful undertaking you are planning, hand-piecing can be time-consuming but beautiful! I hope you will continue here on the quilting board and share your project when you are finished.
By the way, there is a very well-known and highly awarded quilter by the name of Jinny Beyer who hand pieces and hand quilts everything she does; she has a web site and videos you might be checking out.
Jan in VA
Last edited by Jan in VA; 10-19-2013 at 02:57 PM.