What are the differences between wings made for 2 engines and wings for 4 engines?
The differences are surprisingly small, and the implications of two versus four engines are not very great as far as design of the wing is concerned.

Weight implications

The wing is an immensely strong structure - after all, it's able to bear the weight of the entire aircraft, including of course the engines. 

Engines are heavy, but only a small fraction of the weight of the whole aircraft.

A similar aircraft with four engines rather than two will have smaller engines, so the total weight difference will not be large.

Space implications

You need more space to fit two engines on each side, but not a vast amount. For example, the British Aerospace 146 has four remarkably small Lycoming ALF 502 engines. What's more, four small engines can be easier to mount, because they need less ground clearance.

Same wings, more engines

In practice, it's not very difficult to add engines to a wing (for aircraft companies, that is; I don't think I'd be able to do it without seeing someone do it first at least once).

For example, the 747 can optionally carry a fifth engine - obviously, this engine is just being carried, not used, but it shows that simply placing the engine there doesn't present insurmountable aerodynamic or other engineering difficulties.

And then we have the Airbus A330/A340, designed and built in parallel, which are essentially the same aircraft - same fuselage and same wing - one with two engines and one with four. (Actually the -500 and -600 A340 variants do have slightly longer wings, but that's because they are heavier, and nothing to do with the additional engines.)

That's not to say that there are no implications in having additional engines. In this case, wing flutter caused by the outboard engines of the A340 required some stiffening.

And of course, additional engines will require additional fuel and control lines. However, these aren't part of the structure of the wing itself, and are not in themselves likely to have implications for it.