Rent two 2-bedroom timeshares instead. Even paying Redweek prices, it's going to be FAR better than AirBnb -- most of which are illegal here, anyway. And some of them are REALLY sketchy. Like "owner slapped a sheet of plywood across a doorway and ran extension cords to a room with no outlets." That was a post a few months back in one of the Hawaii travel fora.
There are two strategies: splitting your time, and having a home base the entire stay.
I'm partial to the latter. But I see the logic of the former. If you split, you can stay in Volcano. (But be careful about ill-maintained water catchment systems in shady AirBnbs owned by Vlad from Moscow.) Or even stay in Hilo. The drive to the park from Hilo is easy.
Every visitor I've taken to see Kilauea has been happy with a single day at the park. So I think four days is probably overkill. And it's also why I'm partial to having a home-base closer to everything else you want to see. (And for most people, that's Keauhou. Not Waikoloa or Kailua-Kona.)
There is also a timeshare resort (Sea Mountain, Westgate) near Punalu'u black sand beach. It's an unusual choice. But for people who want the volcano, coffee farms, and lots of snorkeling, it makes a LOT of sense. Provisioning will be a chore, though. And there isn't any ticky-tacky tourist-trap stuff nearby.