Having watched several of their matches, I think it's definitely possible that Federer, Djokovic, Nadal and Murray will all reach the SF. IIRC, the last Slam this happened at was the 2012 Australian Open and I honestly would never have thought it could happen again at the start of this year. For me, Murray is most certain to make the SF. His draw has opened up a lot (Kyrgios, Pouille and Wawrinka are all out) and I don't think anyone from his quarter except maybe Tsonga can challenge him. Federer should also reach the SF, though playing M.Zverev in R3, Dimitrov in R4 and Raonic/A.Zverev in the QF is far from easy. However, this is his favorite surface and if he reaches his level of the Halle final, he should beat all of these players. Nadal could make it for the first time since 2011, but he wasn't very convincing today, especially in the 3rd set. Khachanov might be the next big hitter to knock him out and Cilic has a decent shot if he can keep up this level. Djokovic is imo the most vulnerable of the Big 4. He could play Del Potro in R3, who could cause him a lot of trouble if he loses his focus like at RG. It will be a bit easier after that, Sugita/Monfils in R4 and Thiem/Berdych in the QF (though Thiem might just knock him out again
) If I were to make a prediction I'd say Djokovic wouldn't make it, but Roger, Rafa and Andy would.