There is no jailbreak from anything higher than 3.55.
And since your ps3 came new with 4.21, it can not be downgraded to 3.55. Even with a hardware flasher. Which is currently the only way to downgrade from any firmware 3.56 or higher.
Any of those sites like the one you linked are fake. Same with the stupid YouTube videos.
The 3000 and 4000 series PS3s are not hackable.