One reason for using 4 engines for long haul is that the wing structure can actually be lighter as the downward bending weight of the outboard engines can help to counteract the weight of fuel in the centre tanks.
So when the A330 and A340 were designed together the A340 was a long range aircraft and the A330 was a medium range aircraft. The 340 carried much more fuel than the 330 so the wing bending relief of the outboard engines offset the weight of the extra fuel.
The early 330s and 340s had identical wings, just on the 330 there were no engines mounted on the outboard attachment points. Proved very useful for the A330 tanker cos this is where they mounted the refueling pods - the wing structure could take them without any redesign.
Since the early days differences between the 330 and ater 340 models has increased dramatically.