Costa, riding for the home-based Team Emirates, had a four-second lead going into the final day staged on Abu Dhabi's Formula One circuit and finished comfortably in the pack.
Orica-Scott's Ewan of Australia held off Briton Mark Cavendish (Dimension Data) and Germany's Andre Greipel (Lotto Soudal) in a bunch sprint after 143 km of racing.
Former world road champion Costa, 30, had taken control of the race on Saturday when he outsprinted Russian Ilnur Zakarin to win the third stage after an 11km climb to the finish.
(Reporting by Martyn Herman; Editing by Ken Ferris)
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