Chinese President Hu Jintao says China aims to lift the size of its economy to $4 trillion by 2020 -- effectively quadrupling its gross domestic product of five years ago. A $4 trillion economy would give China's 1.3 billion people a per capita income of $3000 by 2020, compared with about $1230 now. Hu, delivering the keynote address at the Fortune Global Forum in Beijing on Monday night, said the 2020 target was a "formidable challenge" that would require an uphill battle. But he told the forum China was committed to seizing the window of opportunity to build a prosperous society. He said economic development was China's central task and top priority. A 2020 gross domestic product of $4 trillion would put China in sight of Japan, which is now the world's second-largest economy ($4.1 trillion) behind the United States ($10 trillion). China, with a GDP of $1.65 trillion last year, now ranks fourth behind Germany, after overtaking the UK and France. "We are deeply aware that China, for a considerably long period of time to come, will remain a developing country," Hu told the forum.