LONDON (Reuters) - American singer Katy Perry has secured her fourth number one in the British singles chart with her latest release "Roar", sales figures showed on Sunday. Already a number one hit in the United States and around the world, the song knocked "Burn" by English singer-songwriter Ellie Goulding off the top spot in its debut week. Perry, whose previous hits include "Firework" and "I Kissed a Girl", sold 179,500 copies in the last week, making it the third fastest selling single of the year, according to the Official Charts Company, which compiles the weekly list. ...