In this terrific question we see the advantages of four engines over two. Namely:
- Engine out capability: Losing an engine means losing only one quarter of the thrust, rather than half.
- Ground clearance.
- Drag: Increased yaw control with four engines in an engine-out scenario means that the vertical stabilizer can be smaller, thus reducing drag.
- The wing is loaded more evenly.
If that is so, then why don't new commercial airliner designs use four engines?