A 51-year-old man has been arrested at an address in the Swindon area under the Terrorism Act 2000, Scotland Yard has said.
The arrest follows an eight-day extension to quiz three men suspected of raising funds for a terrorist group.
The men are being interviewed about the Tamil Tigers - a banned organisation in the UK. The Tigers have fought a 30-year insurgency in Sri Lanka.
The men aged 33, 39 and 46, are to be held until next Tuesday.
The arrests of the first three men were as a result of a series of raids in London and Wales on 29 April.
A Scotland Yard spokesman said the arrests were part of a long-term investigation into support for overseas terrorism.
"This investigation is not linked to al-Qaeda activity," he added.
A fifth man has already been charged in connection with the investigation.
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