Pretty rare to have four different first cousins of those nationalities and full-blooded. More likely to have mixed ethnicity in first cousins (or one or two of the same ethnicities), than four different of full blood. Japanese don't usually marry outside of their race to Polynesians, although there are many Chinese-Hawaiians. Samoans also don't marry much outside their race.
For more on the subject, see Asians in Hawaii Five-0 Remake and Review of Hawaii Five-0 Remake.