4 people? Please don't. rear seats .. even for larger children are sometimes too small. I had driven with two grownup passengers in rear, but only for short distances, and everybody felt very uncomfortable. Even for those in front (as they had to move seats more forward, to gain at least slightly more room behind). Think of twins rear seats less of normal seats, as more like some padded door to boot, folding which allows to put some oversized stuff in car (eg. 4 track wheel set
), not proper seats. Any longer trip with more then just one passenger would be masochistic. Seating of four might be option for emergencies, but i wouldn't willingly consider that for longer trip (which it seems you'll have, judging by more cargo capacity you seek and few days mentioned).