I don't know... I regularly have a 6'1" woman in the back of my Polo GTI, sitting behind a 6' guy in the passenger seat. Both people are comfortable enough. The new Polo is even larger (in length, width and height). So I would say that it would be fine for regular use by 4 adults - even if they're all 6'+

Sure, you're not going to have Crappadore legroom, but then... at least you're not driving a crappadore