It sort of depends upon your driving habits but my guess would be that the four speed would be more fuel efficient. As a general rule shifts are inefficient and the more of them you do the less efficient your driving. In one of our prior fuel crises Saab came out with a recommendation to skip gears, e.g. 1st to 3rd, 2nd to 4th, for better fuel economy. Our US Corvettes do the same thing automatically in order to avoid gas guzzler tax. It's crazy that six speeds are becoming so common at a time of high fuel prices. If common sense prevailed we'd be going back wide ratio 4 speeds.