This is simply not true, with a 2bbl you are running off of all the CFM of the carb all the time but with a 4bbl you are only running off the primaries unless you have your foot in it all the time, the primaries are usually less than half the total CFM of the carb depending on the carb you have. The thermoquad has the biggest difference between the primaries and secondaries and has the potential to get the best fuel economy but you need to find a good one which is not always easy.
That being said, you don't want a mechanical secondary carb.
Like others have mentioned above. The carb is only a small part in getting better fuel economy, your gears, tranny, tire size, exhaust, timming and a lot of other items need to be addressed.