A Metropolitan Police Counter Terrorism Command-led operation resulted in two men - aged 35 and 32 - being held in north Manchester and Bury respectively.
Both men were arrested on suspicion of conspiracy to commit fraud.
Greater Manchester Police (GMP) said two men and two women have been arrested on suspicion of conspiracy to defraud.
In the Met-led operation, officers searched two properties, as well as another house and a business address in Manchester.
Both men arrested were detained for questioning.
GMP said the other arrests were part of a separate investigation, and would not reveal the ages or addresses of the four being detained.
Six people have been arrested following counter-terrorism investigations into online fraud.