Dettori said "She's brilliant, she's honest and she's got an engine. It's just nice to be around her. When you get up on a morning and see your name down to ride her it's a privilege". On 1 July the filly was sent to France for the Group 1 Grand Prix de Saint-Cloud and went down by a short head to the four-year-old colt Waldgeist after a closely contested struggle in the last 100 metres. At the end of the month she contested Britain's most prestigious weight-for-age race, the King George VI and Queen Elizabeth Stakes over on and a half miles at Ascot and finished third behind Poet's Word and Crystal Ocean. In August she ran for the second time in the Yorkshire Oaks and ran second to the three-year-old Sea of Class. As in 2017 the filly ended her campaign with a run in the British Champions Fillies & Mares Stakes in October when she finished second of the eleven runners behind Magical. Coronet's fourth campaign began on 4 May 2019 when she finished fourth behind Defoe in the Jockey Club Stakes at Newmarket. On 30 June she made her second attempt to win the Grand Prix de Saint-Cloud in which she was partnered by Dettori and started at odds of 2.2/1. The favourite was Morgan La Faye (Prix Corrida), while the other five runners were Lah Ti Dar (Middleton Stakes), Marmelo (runner-up in the Melbourne Cup), Aspetar (Grand Prix de Chantilly), Ziyad {Prix Michel Houyvet) and Thundering Blue (Stockholm Cup International). After racing in mid-division Coronet produced a sustained run along the inside rail in the straight and got up in the final strides to win by a short neck and a short head from Ziyad and Lah Ti Dar. John Gosden said "We had planned for this race all year after she just missed out last year... She's a very honest filly. She deserves this first Group 1 success". The mare was back in France for the Prix Jean Romanet over 2000 metres on heavy ground at Deauville on 18 August and was made the 1.7/1 favourite ahead of seven opponents including With You (Prix Rothschild), Wild Illusion, I Can Fly (runner-up in the Queen Elizabeth II Stakes), Worth Waiting (Dahlia Stakes) and Musis Amica (Prix de la Grotte).