Inferno IV is the hardest track in the game, and is unlocked by earning a total of 110 medals and completing "The Greater Crater". Once you've unlocked Inferno IV, you'll need to complete it in under 30 minutes and 500 faults to unlock this achievement.
Extreme tracks can prove very difficult, and it is recommended that players practice as much as possible. You will need a high level of skill to complete this track, and the Pit Viper
is the best option in terms of what bike to use. Below, you can see a video of the track being completed. It also provides commentary for each obstacle along with the best way to complete it. Stay in there and keep practicing!