Swiss officials visited the United States detention centre at Guantánamo Bay, Cuba, last month, the justice ministry has confirmed.
The aim of the trip, it said, was to gather information on inmates Switzerland is considering taking in when they are released.
Justice ministry spokesman Guido Balmer on Sunday declined to confirm a report in the SonntagsZeitung newspaper that the four inmates in question were two Chinese Muslims – or Uighurs – an Uzbek national and a Palestinian.
The human rights organisation Amnesty International welcomed what it considered a "concrete" step. It now hoped the government would be quick to decide who would be taken in and make the necessary preprations.
Switzerland is among a number of European countries, including Ireland, Portugal, France and Germany, that have indicated they might be willing to take in released prisoners who are unable to return to their homeland for fear of persecution.
US President Barack Obama has pledged to close the detention centre, which still houses more than 200 inmates.
swissinfo.ch and agencies
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