Can you get four-person rooftop tents?
The answer is: yes and no. In fact, it's not really a question of weight, but rather of space! Some rooftop tents claim to sleep four people, but this is actually includes two adults and two children. And no, teenagers don’t count as children in terms of the space they take up. There are even rooftop tents that sleep "two adults and three children".
If you like co-sleeping and being close to one another, this is what you're looking for. But be aware that your space will be extremely limited. You won’t be able to sleep diagonally or in a star shape.
If it's family van life that's calling you, we'd recommend you get a camper van with two berths, on which you can fit the Quechua inflatable rooftop tent for vans. That way, you'll have separate bedrooms and so you can get some peace and quiet at night!