For me, four-wheeler means four wheels like those on an office chair. The suitcase stays vertical and is directed by the pull-out grip. Two-wheeler means two fixed wheels, and you pull the suitcase angled 45° on a somewhat longer pull-out grip like a cart. The wheels can be the same size, but I also had ones where the two fixed wheels were larger (2x?). Larger wheels are more forgiving of obstacles as people who use both roller skates, scooters, and bicycles know. But even if they are the same size, the ability of the four wheels to go into different directions causes them to budge on small obstacles, while the two fixed wheels necessarily follow you along.
For me, four-wheeler means four wheels like those on an office chair. The suitcase stays vertical and is directed by the pull-out grip. Two-wheeler means two fixed wheels, and you pull the suitcase angled 45° on a somewhat longer pull-out grip like a cart. The wheels can be the same size, but I also had ones where the two fixed wheels were larger (2x?). Larger wheels are more forgiving of obstacles as people who use both roller skates, scooters, and bicycles know. But even if they are the same size, the ability of the four wheels to go into different directions causes them to budge on small obstacles, while the two fixed wheels necessarily follow you along.