At the start of each season, the tennis world asks whether this will be the year when the grip of the game’s Big Four champions—Roger Federer, Novak Djokovic, Rafael Nadal and Andy Murray—is finally broken. Months later, the season ends and the Big Four is still in charge. But as the Australian Open begins Monday, it’s clear that something is changing. It has to, because one of them is about to exit. Murray,…