VIENNA, Sept 13 (Reuters) - Canadian police have arrested a fourth suspect in connection with the three Austrians linked to al Qaeda who were detained in Vienna on Wednesday, the Austrian Interior Ministry said on Thursday.

The young man detained in the Mauricie region in the province of Quebec had been in contact with the three arrested in Vienna, who are suspected of having posted a video message on the Internet threatening attacks on Austria and Germany, ministry spokesman Rudolf Gollia said.

"The man in Canada was arrested at the same time as the three in Vienna," said Gollia. "They communicated a number of times through online Internet forums and by e-mail."

Gollia said the suspect arrested in Canada was of African descent but declined to give any further details.

The three suspects held in Vienna are a 22-year-old man and his 20-year-old wife and another man of 26, all of whom are second-generation immigrants from Arab countries and hold Austrian passports.

The video message posted in March demanded German and Austrian soldiers leave Afghanistan, but in electronic surveillance over several months police found no concrete indications that attacks were in the offing, the ministry said on Wednesday.

The arrests come just over a week after Germany said it foiled an Islamist militant plan to carry out "massive bomb attacks" on U.S. installations in the country. Germany arrested three men.

The Austrian Interior Ministry said it knew of no links between the German and the Austrian suspects.

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