Over the last 20 years, many airlines have been shifting their fleets and operations towards more efficient twinjet aircraft rather than quadjets. Aircraft like the Airbus A380, A340 and Boeing 747 have given way to the Boeing 777, 787, Airbus A350, and A330. So which airlines operate the most four-engined aircraft?

A quick note before we begin. This list will only include passenger aircraft and exclude freighters. Cargo airlines will not be factored in, despite many still operating the 747-400F. Data will come mainly from Airfleets.net.

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Emirates: 115

With the world's largest Airbus A380 fleet of 115, Dubai-based Emirates is high on the list. Before recent events, we hailed it as the one airline that had the best chances of finding success with the superjumbo. However, with worldwide demand dropping by around 90%, the A380 has become much more of a liability these days.

Lufthansa: 58

German operator Lufthansa has perhaps the most diverse mix of quadjets in the world right now. The airline is listed as having 24 Airbus A340s. This is comprised of 14 A340-300s and 10 A340-600s.

It also has both the Boeing 747-400 and 747-8, eight and 19, respectively.

Finally, while the operator has retired seven of its A380 superjumbos, it has decided to keep the other seven.

Mahan Air: 26

Iran's Mahan Air has a number of quadjets in its stable, from the huge to the very small! It retains two Queens - one 747-400 and one 747-300, both currently parked, and has a fleet of 12 A340s. Of these, five are the -300s and seven the -400s. However, bumping up the numbers is Mahan Air's fleet of no less than 12 teeny tiny quadjets.

Mahan flies the world's largest fleet of BAe 146/Avro RJ aircraft, with 12 aircraft spread across four different models. This little quadjet is often forgotten, but is still four engined and deserves a mention!

Korean Air: 22

While South Korean carrier Korean Air only has 10 A380s, it also has a fair number of 747s in its fleet. The airline is actually one of the few carriers that went for the latest variant of the 747 - the 747-8i. As such, Korean Air has two 747-400s and 10 747-8s. This excludes the freighter variants in the airline's fleet.

Notable mentions

Singapore Airlines has 12 A380s and once upon a time had operated the A340 and 747, while Qatar Airways and Etihad have 10 A380s each.

British Airways most certainly would have made the list had it not retired its 28 747-400 jumbos.

It's very true that quad jets are becoming a thing of the past and have given way to more efficient twinjet widebodies. Right now, it's anyone's guess as to which of the aircraft listed above will see unexpected or early retirements.

For many travelers, advancements in new twinjet cabin comfort will largely compensate for quad jet replacements - even though many hold the A380 in high regard for its quiet cabin and ample space.

But for planespotters, in particular, it must feel like the end of an era, where these giant and magnificent beasts will no longer grace the skies.  However, it's a safe bet that Emirates will have its A380s for at least the next 10 years - that's what it's leadership is saying. Therefore, there is still some time to spot and fly the largest passenger aircraft ever to be built.

Will you miss four-engined aircraft? Or is it high time that these jets are phased out? Let us know your thoughts in the comments!