Considering they're a mix of bloodparrots and convicts, I'd say that's hogwash
I've seen a 6" one, so I know they get bigger than 4"... just not sure how much bigger. My guess would be around the 8" mark, maybe?
There's the potential size of the fish, then there is the potential of the environement. Any parrot should easily reach 8" minimum, but aquarium fish sizes tend to be low-balled because most of the fish end up in less then ideal homes.