You misunderstand. Four engines is more expensive to operate than three which is more expensive to operate than two. Airlines what planes that are cheap to operate. Due to ETOPS regulations the cheapest two engine planes weren't the cheapest to operate on certain routes due to having to take longer routes to stay within x minutes of an airport. So they went one engine up. Three engines made the plane more expensive but this was countered by being able to fly more direct routes.

Four engines were used only on the largest aircraft due to needing all that thrust to fly. Modern engines are powerful enough to only need two and ETOPS regulations have greatly relaxed with increased engine reliability.

When a multi-engine plane loses and engine they never shut down an opposing one if they can avoid it. Planes can fly with asymmetric thrust, it's just not efficient. So while you would never design a plane with asymmetric thrust you don't crash when it happens.