I generally don’t watch the show, so I don’t know all the details about each character’s total airtime, importance to story lines, etc.
However, I would think that Kim and Park have a case if it were an ensemble cast, the top 4 characters all being equal, like the six Friends.
I get the impression, though, that McGarrett and Danny are the two mains, and all the others are basically “Also Starring…”, in which case the pay is commensurate with their position in the show’s hierarchy.
The inequality would arise, I guess, in that the two main characters are white guys, and women/people of colour are in the “Also Starring” category. (I mean, if you can cast a Korean-American woman instead of a native Hawai’an as Kono Kalakaua, you easily could have had, say, an African-American woman as Steve McGarrett.)