If you only have 40 squares, there is a little problem:
Lets do the math
Assuming you sew the 5 squares together with a Ό seam allowance, each finished square will be 4.5.
Dividing 4.5 into 56 (the width of your proposed quilt) = 12.4. This means that you need roughly 12 squares across.
Dividing 4.5" into 64" (the length of your proposed quilt) = 14.2. This means that you need roughly 14 squares down.
To find the number of squares to fill the area, multiply the numbers across and the numbers down. This means that your quilt needs 12 times 14 squares = 168 5 squares to start in order to get the measurement you specified.
With your 40 squares you can do a rectangle of 5x8 squares but it will only be roughly 23x36
Look at the bright side you can go fabric shopping. YEAH!