The squares in the 4- patch unit are larger than the squares in the 9-patch unit.
The width of the finished square in the 9- patch unit is x and the width of the finished squares in the 4- patch unit is approximately 1.414x.
Example: if you were making a 6- inch finised size 9-patch unit, each square would finish at 2 inches wide and the strips would be cut 2-2/2 inches wide.
The finished size of the square in the 4- patch that goes with that would be ( the square root of 2) multiplied by 2. 1.414 * 2 equals 2.828 inches. Cut strip size would be 3.328 inches.
The easiiest way IMO is to draw what you want on graph paper, and then add seam allowances to the pieces.
( pythagorean theoem)