I don't think you could use a 4 inch ruler if you use 5 inch squares of fabric. I have made it using the 3.5 inch ruler and there is just barely enough room to cut as you always end up cutting just a smidge beyond the ruler. If your quilt is sewn with 5 inch squares and is 7 across by 9 down and you use the 3.5 inch ruler you will get a 21 inch by 27 inch when they are sewn back together. Figure out your borders and you can determine your finished size.