* Dubai, Ekaterinburg, Izmir and Sao Paulo in the running
* Successful bid can boost, investment, tourism, prestige
* Some question true costs
PARIS, Nov 27 (Reuters) - Brazil, Russia, Turkey and the United Arab Emirates will learn on Wednesday whether their candidate cities will host the 2020 World Expo, with much at stake in terms of investment, tourism and national pride.
The Brazilian city of Sao Paulo, Ekaterinburg in Russia, Izmir in Turkey and Dubai are in the running to host the next world's fair, which is to be decided in a vote by the 168 members of the Paris-based Bureau International des Expositions.
Voting was due to start at 1330 GMT and was expected to take several hours.
In Dubai, crowds of hundreds of people were waiting for the result in front of big screens outside the Burj Khalifa, the world's tallest building.
World Expos, which participating countries use to showcase technological prowess, culture and architecture, are held every five years for six months. Milan will host the next one in 2015.
The fairs hold huge stakes for would-be hosts, which are prepared to spend billions on infrastructure, hotels, buildings and other preparations to attract millions of tourists.
China said it spent $4.2 billion when Shanghai hosted the last World Expo in 2010, double what it spent at the Beijing Olympics.
Some Chinese media reported the true cost of hosting closer to $58 billion, many times the $164 million profit it reportedly made mainly through ticket sales and corporate sponsorship deals.
While the Shanghai fair pulled in a record 73 million visitors, not all expos prove to be a major tourist draw.
The number of visitors to the 2000 fair in the German city of Hanover were less than half the 40 million expected, leaving a financial shortfall of over a billion euros, according to German media.
But the fairs help the local economy through a surge in public spending and offer the host country a moment in the spotlight on the world stage.
"It's not the matter of a Formula 1 circus that comes in for a week and then leaves," said Brazil's representative, Maria Celina de Azevedo Rodriques.
"It's not a matter where of where you want to be in 2020 but where you want to be after 2020, where you are going to get the best return for your investment," she added.
Dubai, which has lobbied hard to host the 2020 fair, is eager to be the first country in the Middle East to host a World Expo. It has tried to highlight its transport links and says the facilities will be turned into a trade centre after the expo.
With a successful bid for Dubai likely to trigger billions in new investment, concerns have emerged that hosting the fair could create a building boom that could turn to bust - as happened four year ago - if not handled carefully.
Sao Paulo hopes to build on Brazil's success winning the right to host the 2014 World Cup and the 2016 summer Olympics. It aims to develop its site into a new district on the edge of Brazil's biggest city.
Similarly, Ekaterinburg is counting on Russia's experience preparing and hosting the 2014 winter Olympics in Sochi to sway delegates to vote for it.
The Russian government has pledged to stump up whatever funds are necessary, banking on the fair helping to transform the central Russian industrial city a into a hub for technology and innovation.
Turkey's coastal Mediterranean city of Izmir, which lost out to Milan to hold the 2015 world's fair, aims to give a special focus on health and use part of the site for a campus, hospitals and research centres after the expo.
"This not only about Izmir, this is related to the region and global economic growth," Turkish Transport Minister Binali Yildirim told Reuters.
"Millions and millions of visitors will be there during six months."
(Reporting by Leigh Thomas; Additional; reporting by Mirna Sleiman in Dubai; Editing by Ruth Pitchford)