The appointments follow speculation over who would succeed the company's former Contemporary art chief, Tobias Meyer, who left the company at the end of last year.
All four new appointees are experienced specialists who have led Sotheby's teams across the world in recent years, and the promotions bring continuity to the auction house's operations.
"With auction sales that have grown five-fold in the past decade and private sales that have quadrupled since 2010, as well as the development of the S|2 brand, the global gallery arm of Sotheby's Contemporary art department, Sotheby's is uniquely qualified to best serve clients in the marketplace," said chief executive Bill Ruprecht.
"Alex and Cheyenne boast nearly 40 years of experience between them and have strong relationships with virtually every major collector, dealer, advisor and artist in the field."
He was equally confident in his other appointments: "With global sales that grew 21% in 2013 alone, it is clear that Helena and Simon's leadership is a successful formula for our clients and our business.
"Over the course of their combined 44 years at Sotheby's, they have helped bring to market iconic works of art spanning 100 years of art history, including Edvard Munch's The Scream, Alberto Giacometti's Walking Man, and Pablo Picasso's Boy with a Pipe - each achieving prices over $100m and establishing lasting benchmarks in the auction market."