“John Wick: Chapter 4,” the fourth installment in the Keanu Reeves assassin series, debuted with a franchise-best $73.5 million at the box office. That is according to studio estimates Sunday. The Lionsgate film, starring Reeves as the reluctant-but-not-that-reluctant killer John Wick, exceeded both expectations and previous opening weekends in the R-rated franchise. Since first launching in 2014 with “John Wick,” the Chad Stahelski-directed series has steadily grown as a ticket-seller with each sequel. After its disappointing debut last weekend, the superhero sequel “Shazam! Fury of the Gods” slumped to second place with $9.7 million in its second weekend. The Warner Bros. release dropped steeply, tumbling 68% from its launch.