Let's see, I did this, and it works if I understood your problem:

-Create a variable nCounter
-Give it an initaial value of 0
-Value to store is nCounter + 1

-Create an additional group (besides the one on invoice number I suppose) on the expression nCounter < 5, it resets on invoice number

The report shuld do what your trying to do, from here you can add a group footer to nCounter < 5, that only prints when nCounter <=6, and that should be it, HTH

Jorge Haro