SMOC, you have a point, however, it is extremely difficult to get publicly available data, airlines seem very reluctant to release information to make valid comparisons. Perhaps you could point us to an independent source of information.
In the absence of such information we have to make do with the manufacturers data & our own detective work to get a ballpark estimates. In any case all twins outperform quads in terms of fuel burn. There may be certain sectors for which the a four engine is the optimum aircraft (slot restrictions was the Airbus logic for the 380), however, for almost any other sectors twins appear to have significant cost saving.
Sherm's B.O.E calculations seem to agree quite closely with the Boeing data.