You have hit the nail on the head; GP 4000 and other Continental tires of that generation, e.g., Grand Sport Race are noticeably wider than GP 5000. This is allegedly because GP 5000 are designed for wheels with wider internal width.

When I first installed a GP 5000 in my 17 mm internal width wheel, the tire measured just < 23 mm wide. After a few cycles of pre-ride inflation and riding, the tire has stretched to just about 25 mm wide.