Quick-Step Floors rider Trentin completed a personal trio of stage wins in grand tours, adding a victory in the Vuelta to the Tour de France and the Giro d'Italia.
Tom Van Asbroeck of Cannondale-Drapac came third.
Tuesday was Quick-Step's second stage win of the Vuelta after Yves Lampaert took stage two.
Team Sky's Froome stays in the red jersey after finishing in the bunch, leading David de la Cruz (Quick-Step Floors), Nicolas Roche (BMC) and Tejay van Garderen (BMC) by two seconds.
Stage 4 kicked-off with a break of five riders including Johan Le Bon (FDJ), Stéphane Rossetto (Cofidis), Diego Rubio and Nicholas Schultz (Caja Rural-Seguros RGA) and Juan Felipe Osorio (Manzana Postobon) from the six kilometre mark and opened up a gap of 5min 15sec on the rest of the peloton.
Their lead increased to 8 minutes after 80km of racing in very hot conditions before the sprinter's teams decided enough was enough.
Quick-Step Floors, Aqua Blue Sport, UAE Team Emirates and Lotto Soudal all did the heavy lifting as the race entered in the last 100km.
Up front, Rossetto and Rubio broke away from their earlier companions and led by 4min 15sec with 50km to go. But that gap withered under the heat and speed of the peloton and they were finally caught with 8km to go.
A late move from Ricardo Vilela (Manzana Postobon) couldn't derail the sprinters on their way to the bunch gallop won by Trentin.