UPDATE 1-Obama says Iranian candidates very similar

* Obama says two candidates have similar policies

* State Department contacts Twitter over Iran (Recasts with Obama quotes)

By Sue Pleming

WASHINGTON, June 16 (Reuters) - Wading into Iranian politics, President Barack Obama said on Tuesday there appeared to be few policy differences between Iranian President Mahmoud Ahmadinejad and rival Mirhossein Mousavi.

Obama's comments came as thousands of Mousavi supporters took to the streets in Tehran for a fourth day of protests over Friday's disputed Iranian presidential elections.

So far, Obama has been cautious in opining about the candidates, telling reporters earlier on Tuesday he did not want to be seen as "meddling" in Iranian internal affairs.

But in a television interview later on Tuesday, the U.S. president let known his views and said he expected a tough time in any future negotiations with Iran no matter who was in the government.

"The difference between Ahmadinejad and Mousavi in terms of their actual policies may not be as great as has been advertised," he told CNBC.

"Either way we are going to be dealing with an Iranian regime that has historically been hostile to the United States," he added.