What considerations drove the choice of the Solar Impulse engines configuration?
It reduces moments in the wing spar. Major points of axial drag in this design are the wings and the fuselage. You'll see that two engines are close to the fuselage, and the other two are midway along the wing. 
By placing two engines further out, they are also located where the lift is generated, reducing the necessary strength of the wing spar.


Both of the above are especially important since it's such a large wing.


There might also be some possibility of using asymmetrical thrust to ease crosswind landings with this configuration from the yaw from the outboard engines.