Police said a number of charities and businesses in counties Down and Armagh were targeted by men passing counterfeit Â£20 notes on Thursday.
It involved premises in Banbridge, Dromore, Ballynahinch, Moira and Newtownhamilton.
Four men, aged 20, 22, 31 and 45, were arrested on suspicion of fraud in Newtownards town centre.
They were detained when officers on patrol in High Street stopped a black vehicle after they noticed "suspicious" activity.
The men are currently assisting police with their inquiries.
Four men have been arrested in Newtownards, County Down, in connection with a fake banknotes investigation.