Sunde, speaking in a video clip posted on the internet after the verdict was delivered, said: "We can't pay and we won't pay.
He held up a hand-scribbled "I owe U" note , saying: "This is as close as you will get to having money from us".
Defence lawyers had argued that the four men were innocent because The Pirate Bay does not actually host copyright-protected material.
Instead, it provides a forum for users to download content through "torrent files", which allow people to transfer files from several different users.
Ross Anderson, professor of security engineering at Cambridge University, told Al Jazeera the quartet had found what they believed was a "legal loophole, because they simply pointed to the content, they didn't upload it".
He said the defendants could potentially take their case to the European Court of Justice, which has said the law must be predictable.
"Why should they be sent to prison for talking about the existence of illegal content when [some] journalists point to books that are banned?"
"We see more and more of these surprises, because the law is increasingly out of date with technology."
But the Stockholm district court said the defendants were guilty of helping users breach copyright law by "providing a website with ... sophisticated search functions, simple download and storage capabilities, and through the tracker linked to the website".
John Kennedy, head of the International Federation of the Phonographic Industry, said the verdict was "good news for everyone ... who is making a living or a business from creative activity and who needs to know their rights will be protected by law".
But Mark Mulligan, a music analyst from Forrester, a research company, was doubtful the verdict would act as a strong deterrent.
"Every time you get rid of one, another bigger one pops up. Napster went, and then up came a whole host of others.
"The problem of file-sharing just keeps growing year on year, and it's increasingly difficult for the industry to do anything about it," he said.
The case was opened in May 2006, after police raided the website's locations in Sweden, seizing servers and computer equipment and temporarily shutting down the site.
It triggered debate about file-sharing in Sweden, where many defend the right to swap songs and movies freely on the internet.
The defendants previously said they would appeal if they were found guilty.