Just to be picky, really picky.. the wing bending moment is not just caused by lift.. its caused by the fuselage being pulled down by gravity whilst the wing is maintained by a pretty uniformly distributed lift force.
If you think of the wing with no engines as a simple cantilever, like a pencil held between two fingers it moves up and down quite a lot as you apply load to one end....then try to stop it moving and you soon see how it snaps.
However add a weight at the mid point and the movement is reduced and the force needed to restrain it is reduced, hence the tendency to snap is also reduced...add another weight further out and its going to move even less.....and less force is needed to restrain it is reduced and so even likely to snap. So for an equal strucutral strength you can reduce the dead weight of structure by hanging engines off it..
That's the thought behind the use of engines spread across the wing and also the thought behind having fule in the wings spread across the majority of the span.
My guess for the A340/A330 divergence is that for an equal strucutral weight the A340 can carry a higher fuel load.. hence longer range. As the A330 is not destined for such long sectors the additional fuel is not needed and hence the 4 engines are not helpful.
Detailed examination of the 767/ 777 dry weights versus fully loaded weights may reveal an increased efficency in the A340 solution.