Among the four was Syrian refugee Sara Mardini, who gained international attention when she and her sister - both competitive swimmers - reached the Greek island of Lesbos in 2015, swimming for their lives after the inflatable boat they had left Turkey in started taking on water.
The four face charges including belonging to a criminal organization, money laundering, espionage and breaches of immigration laws. They deny wrongdoing.
Mardini started a new life in Germany but later returned to Lesbos, a key point of entry for migrants trying to reach Europe, to help others arriving there and worked with the Emergency Response Center International charity. Her arrest on severe criminal charges prompted international concern.
Mardini's sister Yusra was a member of the refugee team at the 2016 Rio de Janeiro Summer Olympics.
The charity members' lawyer, Haris Petsikas, said all four were released from prison pending their trial after paying bail. He said Mardini and Sean Binder of Germany are free to leave Greece but have no immediate plans to do so. The other two suspects, both Greeks, also paid bail but were forbidden to leave the country.
Petsikas said the trial was not expected to start for at least six to eight months.
Greek police claimed the four allegedly facilitated the illegal entry of migrants to Greece for profit, to raise donations or subsidies for the charity, had advance notification of smuggling boats' routes and numbers of passengers and illegally monitored Greek and European coast guard radio traffic.
Most migrants reaching Lesbos by sea make the journey in leaky boats after paying a high price to smuggling gangs.
Petsikas said the four being released from pre-trial detention was a good sign.
"The evidence that was provided (to authorities) negates the charges," he said.
Copyright 2019 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.