The -300 has the same wing as the 1 and 200 series. The -400 has another 11' of wing span over the Classic in addition to the winglets. There are also other aerodynamic and structural differences.
The burn on a -400 is on average 16% less burn than a Classic. Some of this will be down to the engines, but most aerodynamic. I think the winglets are more than just a marketing ploy.