Three of the men, all in their 20s, were allowed to go on Sunday evening, hours after the fourth was freed.
All four must remain available for any further police enquiries.
The men were seized Saturday night and their apartments searched, said police in Cologne, near Bonn where the festivities on Monday to commemorate German reunification were due to be attended by Chancellor Angela Merkel.
The four were active in Islamist circles, according to authorities.
Police said Sunday that “no arms or other dangerous objects,” had been uncovered during searches, and that there was no apparent threat which required their further detention ahead of the celebrations to mark the 1990 reunification of East and West Germany.
“There is no proof nor links between the accused and terrorist groups nor concrete preparation for an attack,” said a spokesman for the federal prosecutor in the southwestern Karlsruhe district.
Following the arrests on Saturday the police had said in a statement that the four were arrested “as it could not be excluded that a crime was being prepared.”
Alongside Merkel, Germany’s President Christian Wulff and other notable dignitaries are due to attend Monday’s ceremony.