Striker- I use the 6" counterweight with all sizes of pads, including 4".
Note that the weights were made to go with the original PC backing plate with (only) one sheet of sandpaper stuck to it and zero applied pressure...quite different from aftermarket plates, with foam pads wet with product, and plenty of applied pressure.
Most people don't notice a huge difference, and I doubt that it *really* matters which weight you use. So yeah...maybe you wasted a few bucks.
Odd that your PC didn't come with *any* counterweight...both of mine came with one (5" on the one, 6" on the other).