The 4x4x48 challenge is an endurance challenge created by David Goggins.
The purpose of the challenge is to run 4 miles every 4 hours for 48 hours. You end up running 48 miles in 48 hours and the challenge is as much mental as it is physical (not to mention the issues of sleep either!)
In his book "Can't Hurt Me" - Goggins describes several workouts he does throughout the year.
One of them is the 4x4x48 challenge.
Every 4 hours
for 48 hours.
It's as much of a challenging sleep workout (you don't get much time to rest) as it is physically difficult to run that much throughout the day.
For those that attempt it - it's eye-opening for the levels that it pushes you to and even seasoned endurance athletes have issues with the unique format that this challenge brings - spreading 48 miles over 2 (sometimes 3) days.
Joel Runyon did the 4x4x48 challenge with Eric Anderson of Ocean Lab in Austin, TX
Billy Yang takes on the 4x4x48 challenge
David Goggins is a former Navy SEAL, endurance athlete and motivation speaker. He is guaranteed to pump you up and make you want to run through a brick wall.
Other challenges you might want to read about