Kim Dotcom, founder of the online filesharing website Megaupload, is remanded in custody Reuters

The file-sharing multimillionaire, Kim Dotcom, is facing a further four weeks in custody after a judge in in New Zealand denied him bail.

Judge David McNaughton ruled that the 38-year old German founder of Megaupload.com could try to flee the country if he were released from jail pending an extradition hearing on 22 February in connection with charges in the US of facilitating millions of illegal downloads of films, music and other content, costing copyright holders at least $500m (£320m) in lost revenue.

"With sufficient determination and financial resources, flight risk remains a real and significant possibility, which I cannot discount, and bail is declined," the judge said.

The discovery of unlicensed and illegal guns at Dotcom's rented mansion north of Auckland when he was arrested with three others on Saturday could make it more likely that Dotcom would try to escape to Germany, where he would be safe from extradition, the judge said.

Dotcom's lawyer, Paul Davison, said he was "very disappointed", and would appeal against the decision immediately.

"The judge has agreed with much of what we have submitted but he has taken a different view on the issue of flight risk," Davison said.

Dotcom's legal team repeated that he denies the charges, adding he was suffering from diabetes and hypertension, and receiving treatment for a slipped disc.

In the Netherlands, the prosecutor's office said Dutch police last Friday arrested a fifth suspect, identified in the US as Andrus Nomm, 32, a citizen of Estonia and a resident of both Turkey and Estonia. He has been detained for 60 days pending an extradition request.

Megaupload.com became the 13th most-visited site on the internet, according to the FBI indictment, attracting more than 180 million registered users. Premium subscribers to the site, which allowed unhindered downloading of films, TV and music that had been uploaded by others, delivered an income of $150m while adverts delivered a further $25m.

The indictment puts it: "Since at least September 2005, Megaupload.com has been used by the defendants and other members and associates … to willfully reproduce and distribute many millions of infringing copies of copyrighted works, including motion pictures, television programmes, musical recordings, electronic books, images, video games and other computer software."

It names films, including parts of the Harry Potter and Lord of the Rings franchises, among those whose copyright were infringed.

Dotcom's neighbours and business associates have been describing the business approach of a man once fêted in Germany as a leading light of the internet business boom.

One former business associate, speaking under condition of anonymity, told the Guardian Dotcom was "a very smart guy, very clever, very quick on the uptake, very driven, very pedantic".

While he was trustworthy – "within reason", she said – Dotcom at times showed symptoms of "anger issues" and too often "thought money could fix everything". "He likes playing God," she added.

On Tuesday the Guardian revealed the contents of an email Dotcom sent to neighbours in 2010 joking about his reputation as a hacker with a criminal past and a party-loving hedonist.

He joked: "My close personal relations with other (far worse) criminals can help you whenever you have to deal with a nasty Neighbor". He signed off: "If you feel like it come over for coffee sometimes. And don't forget to bring the cocaine (joke). All the best, Kim."

Late last year, Dotcom claimed he had turned over a new leaf.

"I made mistakes when I was young and I paid the price," he wrote on a website about file-sharing. "I am 37 years old now, I am married, I have three adorable children with two more on the way (twin girls – yeah) and I know that I am not a bad person. I have grown and I have learned."