Fractional ownership of aircraft involves an individual or corporation who pays an upfront equity share for the cost of an aircraft. If four parties are involved, a partner would pay one-fourth of the aircraft price (a "quarter share"). That partner is now an equity owner in that aircraft and can sell the equity position if necessary. This also entitles the new owner to a certain number of hours of flight time on that aircraft, or any comparable aircraft in the fleet. Additional fees include monthly management fees and incidentals such as catering and ground transportation. In the United States, fractional-ownership operations may be regulated by either FAA part 91 or part 135.
Though the early Lockheed Jetstar had four, most production business jets have two jet engines, mostly rear-mounted podded engine. If mounted below their low wing, it wouldn't allow sufficient engine clearance without a too long landing gear. The HondaJet is the exception with its over the wing engine pods. Dassault Falcon still builds three-engine models derived from the Falcon 50, and the very light jet market has seen several single-engine design concepts and the introduction of the Cirrus Vision SF50 in 2016.