Chinese President Hu Jintao says China aims to lift the size of its economy to $4 trillion by 2020 -- an effectively quadrupling of its gross domestic product of five years ago.

Hu, right, at the Fortune Global Forum with Time Warner CEO Richard Parsons.[cnn.com]

Hu, delivering the keynote address at the Fortune Global Forum in Beijing, 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, which was opened by Time Warner Chairman and CEO, Richard Parsons.

Like Fortune magazine, CNN is part of the Time Warner group.

"We must focus on economic development as our central task, making development our top priority and facilitating and all-round progress in economic, political and cultural aspects and in the building of a harmonious society," Hu said.

After a stunning growth rate of 9.5 percent for the past 15 months and 9.3 percent for the year before that, most analysts expect China's government-induced cooldown finally will start to have an effect later this year.

They expect growth this year will slow to about 8 percent -- still better than any other significant economy in Asia and more than enough to keep China's mantle as a global engine of expansion.

Hu told the international business audience at the forum China would continue to work hard to open up to overseas investors.

"China will unswervingly pursue a basic policy of opening up to the rest of the world and will further pursue our economic and technical cooperation with the rest of the world," Hu said.

"We are certainly willing to create an even better climate for foreign businesses to make investment and trade with China," he told the business forum.

But there is friction over China's currency exchange rate and its burgeoning textile exports to markets such as the United States and Europe.

Critics claim China gets an unfair trade advantage by keeping the yuan pegged at 8.28 to the dollar, instead of letting it strengthen.

Vice Premier Zeng Peiyan, addressing the Fortune Global Forum on Tuesday, said China would push forward steadily with reform of the yuan. His remarks followed those of Premier Wen Jiabao, who said on Monday that Beijing would not bow to outside pressure for a rise in the currency.

Hu's forum speech on Monday night made no mention of the trade row over textiles. The United States said last week it may restrict imports of three kinds of clothing from China, and the EU is considering similar curbs.