While the wing bending moment relief provided by the outboard engines is real and quantifiable feature, it is only one factor in integrating an airplane. Other Twin vs Quad issues come into play.
The latest versions of the 777-300ER and A340-600 have almost the same payload - range. Comparing airplane weights and fuel volume should give you an idea of the relative efficiency. You can draw your own conclusions.
773ER A346
Operating Empty Weight (tonne) 166.8 181.9
Max Takeoff Weight (tonne) 351.5 380.0
Fuel Volume (USG) 47,890 51,379