British police arrested a 25-year-old man for alleged terror offences on Tuesday, after three others from the same town were detained over a reported plot to kill Prime Minister Gordon Brown.
The latest arrest was made in Blackburn, northwest England, also home to the three men in their early 20s detained on August 14: two at Manchester Airport as they prepared to fly to Finland, and the third in the nearby town of Accrington.
"The arrest is part of an ongoing investigation by Lancashire Constabulary and Greater Manchester Counter Terrorism Unit," said a police spokeswoman.
Police sources said the arrest is linked to a probe into threats to kill Brown. While no concrete planning was in place, there were calls for the British premier's death on an extremist jihad website in January.
Following the new arrest detectives were searching premises including a flat behind a Muslim butcher's shop, while a police van was parked outside an Islamic bookshop.
"This is a very sensitive time for all our communities and we are working hard to ensure everyone is kept up to date with as much information as we can give," said police chief Andy Rhodes.
"We will endeavour to carry out any searches as quickly as possible to minimise impact on the area concerned.
One local resident, who declined to be named, said: "I thought the police would be finished by now, but it's gone on another week. I thought they would have arrested the man at the same time."
The man arrested on Tuesday lived on his own, the resident said.
Media reports on Friday said the first three arrests were made after a group calling itself Al-Qaeda in Britain posted a statement on a jihadi website which also demanded the withdrawal of British troops in Iraq and Afghanistan.
British police have travelled to Scandinavia as part of their investigation, reports said.
Officers have until Thursday to charge or release the men, or seek a further extension of their custody time-limit.