There is a slight difference between the 5" and 6" counterweights with my 6" pad set. Enough of a difference that it's a no brainer to change the counterweight.
But with the 4" pad set and 5" counterweight, the operation is even more smooth. Haven't tested it, but it seems like you have more power with the machine too because it would not bog down as quickly, making defect reduction more effective.