Hoffpauir also tied the team record for homers in a game set by Wade Rowdon on June 9, 1987 against Oklahoma City. The last PCL player with four homers in a game was Las Vegas' Eddie Williams on April 22, 1998 against Calgary.
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"I've been working on trying to stay short on my swing and not try to do too much, and the patience and practice paid off," Hoffpauir said. "I never hit more than two homers in a game and after the third one, the guys were starting to get on me a bit.
The left-handed hitter, who has 19 home runs this season, slugged a fastball to center field in the first inning, belted a changeup over the fence in right in the third, smacked another fastball to left-center in the sixth and ripped a slider to center leading off the eighth.
"It was a lot of fun," Hoffpauir said. "And Round Rock isn't too far from home [Fort Worth], so I had a lot of family here. That made it all the more special."
He came to bat during a seven-run ninth with a chance to tie Pete Schneider's 1923 all-time PCL mark with a fifth homer but flied to right.
"I don't think the pitchers made bad pitches tonight," he said, "I was just able to hit them."
Hoffpauir, who spent the first five weeks of the season on the disabled list with a strained oblique muscle, is hitting everything these days. He is 37-for-78 (.474) during a 13-game hitting streak that knocked his average up to .374.
His season included 19 games in the Major Leagues with Chicago, where he batted .378 (14-for-37) before returning to Iowa in June.
"A lot of great things have happened this season," he said. "Once I got a taste of the big leagues, I knew that's where I wanted to be. I struck out more than I wanted to, but it was a great experience.
"Being sent back here gave me motivation to get back there."
Felix Pie, Matt Craig, Jason Dubois and starting pitcher Mitch Atkins also went deep for the Cubs (70-50), who broke the club record of seven homers in a game, set on June 9, 1987 against Oklahoma City and equaled on Aug. 15, 1999 at Omaha.
Atkins (6-0) won his 11th straight start over two levels after allowing three runs on eight hits over 6 1/3 innings. He walked three, fanned four and remained unbeaten since June 9.
Starter Joshua Miller (7-7) surrendered six runs on nine hits over seven frames, striking out two without a walk, for the Express (55-66).
Ray Sadler, Mark Saccomanno and Yordany Ramirez hit solo homers for Round Rock, but J.R. House hit into a triple play for the second time in less than three weeks.