Why do modern airliners have only two engines? 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?
Engines are expensive, maintenance-intensive and heavy. You only need 2 to keep a large plane in the air.

In the days of the 747 regulations required at least 3 engines if you ventured more than 60 minutes of flying time away from a suitable diversion airport (in case the second one also failed). However as reliability of engines increased this got extended with ETOPS certificates.