The Norwegian 4x4 protocol involves 4 minutes of maximal sustainable intensity (85-95% HRmax) followed by 3 minutes of lower intensity (~70% HRmax), repeated 4 times.

“4 minutes at the maximum intensity you can sustain (for a given pace, usually around 85-95% HRmax) • 3 minutes at a lower intensity (~70% HRmax) • Repeat 4x”

“High-Intensity Training: Once a week, perform a high-intensity workout like the "Norwegian 4x4" routine.”

“the clinical workout also known as the Norwegian 4x4 which I like to call it because the Norwegian ski team often uses this hit protocol for their training it's a 4minute interval where you're going as intense as you can for four minutes and then you have light recovery for three minutes so you're going very very light getting your heart rate down and that's repeated four times so it's about a 25 minute workout”

“people are doing four different intervals so they're doing for for four minutes they are exercising typically on a stationary bike perhaps a rowing machine you could you could do it on a treadmill as well but a lot a lot of the the protocols were on on a bike and you're doing for for the four-minute interval you're you're going at an intensity that you can maintain for that entire four minutes that's pretty hard so it's it's pretty pretty much a sustainable hard intensity for 4 minutes it's obviously not going to be maximal but you're going as hard as you can for and as hard as you can sustain for that 4 minutes and then you follow that by a three minute recovery period which is very light right you want your heart rate to come down you want to clear your lactate and then you go back into the four minute interval so that's a 4 by 4 you drink it four times”