The four suspects are being held “on suspicion of being concerned in the commission, preparation and instigation of acts of terrorism,” West Midlands police said in a statement.
Police did not name the four but said they were aged between 22 and 32 and came from Birmingham, Ipswich and Northampton in England and Powys in Wales.
"The arrests were pre-planned and intelligence-led; there was no threat to the public's safety," the statement said, adding that raids were also being carried out in "a number of properties".
The defence ministry said: “We can confirm that a number of serving members of the Army have been arrested under the Terrorism Act for being associated with a proscribed far-right group.”
National Action became the first far-right group to be outlawed by the government in December last year, six months after the assassination of MP Jo Cox by a far-right sympathiser, Thomas Mair.
