How does it work?
A 4×4 is well known climbing circuit workout. Pick 4 boulders at your flash level (got them first try). Then pick the first boulder and climb it 4 times in a row without rest. When you are done with the first boulder, rest for 2 minutes and then repeat on the remaining three.
How does it improve your climbing?
A 4×4 trains your power endurance (near maximal effort for a sustained period of time). This makes up a lot of bouldering, so it is an extremely good area to spend time on developing.