Sew your 4 strips together, press.
Measure across the width of these sewn sets at s few different points and average those measurements. (will be close to 8.5")
Cut your squares this size.
Remember that a square has FOUR EQUAL SIDES, so if the strips are 8.75" wide, you would cut them in 8.75" chunks to make them square........or if 8.25" wide, then cut at 8.25" chucks for a square.
Jan in VA