By Jonathan Stempel and David Randall NEW YORK (Reuters) - Warren Buffett on Saturday forcefully defended Berkshire Hathaway Inc's decision to invest heavily in stocks of companies such as Apple Inc as he labors through a four-year drought since his last major acquisition of a company. In his widely-read annual letter to Berkshire shareholders, the 89-year-old Buffett also assured that his conglomerate is "100% prepared" for the eventual departures of him and Charlie Munger, his 96-year-old vice chairman.