The space shuttle had solid rocket boosters and airforce jets have had drop tanks for 50+ years -so the concept of dumping surplus weight (and drag) when in flight is very well established.
Today’s invention extends that to engines. Most airliners can cruise quite efficiently and safely using only two of their four engines (once takeoff has been completed).
The idea would be to use all four jets to gain altitude and then jettison the outer two. This would greatly decrease the weight of the vehicle but most importantly the drag would be similarly cut, allowing the plane to fly very much farther (if slower) using its usual fuel load.
The dropped engines would have parachutes and transponders attached, enabling their recovery and reuse. A small retained fuel supply could even be used to fire the falling engine up just before landing, in order to provide a retro-thrust effect and cushion the impact.
When the newly 2-engined plane landed, it would be refitted with another two temporary propulsion units during turnaround.
(If this is too hair-raising to be seriously considered, how about mounting an engine or two beneath the fuselage so that they could be withdrawn from the airstream, like the undercarriage is?)