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.

The 1996 world indoor champion, a bowls shop retailer, will again concentrate on his own competitive play.
Gourlay said: "For me, the head coach role was only ever going to be for the Glasgow Commonwealth Games.
"And I don't think that anything will ever beat that experience of being coach at a home Games."
Under Gourlay, Scotland returned from the Adelaide World Championships two years ago with six medals, including three golds.
The integrated team of mainstream and para bowlers went on to win four medals, three of them gold, at Glasgow 2014.
Gourlay expressed disappointment that, despite achieving twice as many medals as they targeted at the home Games, the women's team did not win any.
He also admitted standing down was "an incredibly hard decision to make because the squad and I have grown really close over the past three years" and he would miss working with the players.
"One of the highlights for me was the recognition and respect that bowls got at the Games, not only through the media but with other sports," said Gourlay.
"We've got to be a professional sport if we want to be taken seriously and we've taken a couple of steps on the performance ladder."
Double Glasgow gold medallist and Scottish Sportsperson of 2014 Alex Marshall paid tribute to the outgoing coach and team manager Ricky Taylor.
"With David as coach, we had far more training camps, test series, seminars, team-building days, psychology sessions with the full support of the sportscotland institute of sport, and lots of practice sessions at Kelvingrove," he said.
"David and Ricky built a programme based on the team's shots on every rink in every direction at Kelvingrove.
"So, when we competed at the Games, whatever green or rink we found ourselves on, we had a great idea how it was going to perform before we went on it. That made a massive difference."
Bowls Scotland believes that Gourlay helped change the sport's image - and the culture of performance bowling - in Scotland.
Chief executive Alan McMillan added: "Whilst I am disappointed that David is leaving the post, he has left performance bowls in a better place and set a benchmark for anyone who has aspirations to become a world-class player. "
Four men have been arrested in Newtownards, County Down, in connection with a fake banknotes investigation.
[NEXT_CONCEPT]
David Gourlay is to step down from his role as Bowls Scotland head coach after leading his team to four medals at this summer's Commonwealth Games in Glasgow.