The two inner engines are in the expected position. Keeping the engines as close to the fuselage as possible has two main benefits:
- Less adverse yaw in the case of an engine failure
- Less stress on the wing on the ground and during landing from the engine weight
The outboard engines are certainly unconventional. Most aircraft with four engines, expecially propeller-driven aircraft, tend to keep them all as close to the fuselage as possible. Some considerations that may have affected this decision:
- Less stress on the wing in flight. The weight of the engine is offset by the lift produced on the wing. Large aircraft generally use the fuel in the wing tanks last for this reason.
- Putting the engine at a point that is already structurally important can save some weight by combining the structure. In this case:
- The wing angles upwards here, so there would probably be a bit more structure to support this bend
- The outboard landing gear is also here
- This could also mean less weight in wiring to hook up the engines to power
It is interesting to note the design of the Solar Impulse 2. There seems to be a different compromise between the two cases above, with both engines on each wing placed together roughly in the middle of the span. This seems to favor the case of saving structural weight.