The Roberts home run highlighted a five-run inning, quickly brought the Diamondbacks back from an early deficit, and sent the teams to a fifth and deciding game in their National League Division Series.
Milwaukee will host the final game Friday. The winner of that contest will meet either Philadelphia or St. Louis for the league championship. St. Louis defeated Philadelphia Wednesday to forge a 2-2 tie in their series.
Randy Wolf, making the third playoff start in his 12-year career, surrendered the big opening inning and the Diamondbacks eventually produced four homers among their 13 hits.
Willie Bloomquist singled to start the Arizona first, went to second on a walk to Justin Upton and moved on to third on a base hit by Miguel Montero. That brought up Paul Goldschmidt, who hit a grand slam homer in Arizona's 8-1 victory Tuesday. Goldschmidt was unable to repeat that performance, striking out looking instead, but Roberts followed with a blast to left on a 2-1 pitch.
Chris Young then hit a first-inning homer as well and Young scored along with Goldschmidt in the third on a single by Collin Cowgill.
Aaron Hill added a solo homer for the Diamondbacks in the sixth and Young hit his second blast of the game in the seventh, a two-run shot.
All of that offense was enough for Arizona to overcome single runs for Milwaukee in four of the first six innings along with a two-run homer by Carlos Gomez in the eighth.
The pitching matchup for Game 5 is expected to be a repeat of the opening game of the series -- Yovani Gallardo for Milwaukee and Ian Kennedy for Arizona. Gallardo won that one in a 4-1 victory for the Brewers.