The rumor is a larger display due to a bezel shrink, not because of a physically larger watch. So no change to the bands..
Technically the most concrete rumor was from Ming Kuo with a 15% larger display, which was never actually specified what that meant in terms of reducing the bezels or enlarging the display. Either way, my point behind the comment of retaining the band port was the concern from others that if Apple changed that, that would mean other physical changes to follow, but obviously that’s not suspected to be a major concern at this point.
Another member in the Apple Watch forum claimed that he observed a Series 4 in the Cupertino area and basically it was what a Series 3 was, but just larger. I expect that to be true of the Series 4, as Apple is not really known to make drastic changes to their hardware.