The 53-year-old man, who had an arrest warrant out, was caught upon arriving at one of Germany’s busiest airports, officials said.
Police had arrested a 39-year-old woman and two men, aged 33 and 49, on Wednesday evening on the same suspicions.
Another man who had been arrested was later released.
Four suspects now sit in detention.
Officials accuse the suspects of preparing a serious act of violent state subversion, as well as possession of illegal weapons.
Additional details, including a possible Islamist motive or the nationalities of the suspects, were not given by officials.
The man arrested at the airport was the brother of the 49-year-old arrested Wednesday, prosecutors said. He is suspected of being involved in either the transfer or storage of military weapons.