The charges in the five-week trial at Maidstone Crown Court came after a fifth man who confessed to the smuggling plot turned supergrass.
Ronald Johnson, 52, was the leader of the gang arrested at the end of 1991 as part of Operation Theft, set up by Customs in conjunction with Dutch police, the jury was told.
Johnson and Ronald Main, 44, both of Storrington, West Sussex, Dean Akehurst, 29, of Worthing, West Sussex, and Brian Kemp 25, of Herne Bay, Kent, denied smuggling and conspiring to supply drugs. All were found guilty on both charges, apart from Kemp who was cleared of smuggling.
Benno Blankenstein, 57, from the Netherlands, had admitted his involvement in the operation and provided information against the others, the court was told. He brought suites into Britain packed with millions of pounds worth of drugs.
He slipped past Customs six times, but the gang was arrested after the seventh consignment, worth pounds 23m, was discovered in Blankenstein's lorry at Sheerness docks, Kent.
When police raided Johnson's house they found pounds 570,000 in cash and 100,000 Ecstasy tablets weighing 25 kilos (55lbs). Johnson, Main, Akehurst and Kemp were arrested that day, while Dutch police held 29 people and seized 577 kilos of Ecstasy tablets, 400 kilos of Ecstasy powder, and pounds 3m in cash.
The gang wanted to corner the Ecstasy market in Britain and flood clubs and rave parties with the drug, the prosecution said. Main and Johnson both claimed that they were used-car dealers who knew nothing about the drugs trade.
All four defendants were remanded in custody for a month to prepare for a drug trafficking offences order, where any assets related to the smuggling plot will be seized. They are expected to be sentenced along with Blankenstein shortly afterwards.
A biochemist was yesterday jailed for seven years for his role in a scheme to distribute Ecstasy worth pounds 7m.
Paul Ring, 31, a London University graduate, helped to make liquid Ecstasy and convert it into tablet form, Francis Sheridan, for the prosecution, told the Old Bailey. Ring, who received at least pounds 10,000 for his laboratory work in this country and in the Netherlands was also ordered to forfeit nearly pounds 2,000 - the only assets he has left.
Ring, of Windsor, Berkshire, admitted conspiracy to produce Ecstasy and possession of the drug. Other members of the gang were jailed for up to 10 years last month. When police grabbed their pounds 7m haul it was the UK's largest Ecstasy seizure.