Two big reasons for 4 doors. First and foremost, it’s a real pain getting larger items in and out of the back seat - boxes, etc.
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I rarely have more than one passenger. But I do like to put groceries on the rear seat.
But, mostly I'm carrying stacks of stuff to and fro, bags, boxes, tools - well, just a host of things I have to move around - so those extra doors come in mighty handy quite often.