Police said that detonators fired on explosive devices made of hydrogen peroxide and wheat flour, but the main charges failed to explode.
The attempted attacks came two weeks after four suicide bombers killed 52 people in the British capital.
The jury is still considering verdicts against two other men facing the same charges.
All six defendants denied conspiracy to murder and conspiracy to cause explosions likely to endanger life.
Hoax claim
The convicted four claimed that the bombings were a hoax designed as a protest against Britain's involvement in Iraq, but were not intended to kill.
Eritrean-born Ibrahim, the self-confessed bomb-maker, said the bombs were deliberately designed not to explode but only to go "pop".
Omar, Mohammed and Osman tried to set off their devices aboard underground trains, while Ibrahim's device failed to explode when he attempted to activate it on a double-decker bus.
The explosives were packed in plastic tubs, with screws, bolts and other pieces of metal taped to the outside as shrapnel. The detonators contained triacetone triperoxide (TATP), an explosive used by the July 7 bombers.
Police said scientific tests on the devices proved that they were all viable. They did not know why they did not work.
CCTV footage
Much of the prosecution's case was based on eyewitness accounts as well as closed circuit television footage from the targeted underground trains and the bus.
In one piece of footage, Mohammed attempts to detonate his charge with his backpack facing a mother and young child.
Moments, later passengers are seen running to the other side of a carriage, while an off-duty firefighter remonstrates with Mohammed.
Police believe that the planning for the attack started after Ibrahim returned to Britain from a trip to Pakistan in March 2005. He was in the country at the same time as two of the July 7 bombers - Shezhad Tanweer and Mohammed Sidique Khan - but officials do not know if they ever met.
They believe the transit system was not the original target, but was chosen following the successful attacks two weeks earlier. Their original target is unknown.