I know you can ride the rafts solo. They put different parties together. I don't know how many they hold, but they fill them up. So if they did hold 4 people, you'd just have to wait for a party of 3 or 2.
Yeah, I'm slightly disappointed about going now, but I'm going to try it out.
You are going on your trip before me, so if you can, please post how it goes.