The trial judge said the men had wanted to replicate the attacks of July 7 and that both groups were under the direction of al-Qaeda.
The judge said: "I have no doubt they were part of an al-Qaeda inspired and controlled sequence of attacks."
"It's clear that at least 50 people would have died, hundreds would have been wounded, thousands would have had their lives permanently damaged," he told the court.
Prosecutors also told Woolwich Crown Court in London that they would seek a second trial against Manfo Kwaku Asiedu and Adel Yahya, after the jury failed to reach a verdict against them over the same charge.
Judge Adrian Fulford dismissed the jury on Tuesday after they said they could not agree on a verdict for Asiedu and Yahya.
All six defendants denied the charges, saying the devices were duds and their actions a protest against the Iraq war.
But police and prosecutors said scientific tests proved the bombs were all viable. They do not know why they did not work.
Deputy Assistant Commissioner Peter Clarke, the Metropolitan Police anti-terrorism chief, said the four convicted plotters "set out to replicate the horrors that had been inflicted on Londoners on July 7, 2005."
"The convictions show that the jury rejected the blatant, indeed ridiculous, lies told by these defendants in a futile attempt to escape justice," he said