The Associated Press

CINCINNATI — The Reds are slowly getting their rotation back in shape. Still, they proved in a 4-1 loss to the Chicago Cubs on Wednesday that they have little room for error.

Carlos Pena and Aramis Ramirez hit homers on consecutive pitches after shortstop Paul Janish’s error in the fourth inning, costing the Reds their shot at a three-game sweep.

Right-handed reliever Jose Arredondo also committed a run-scoring balk that manager Dusty Baker said was the result of a miscommunication.

“We’ve got to tighten up our game,” Baker said before the Reds headed west on a six-game trip to San Francisco and Los Angeles. “We’re real close.”

Ryan Dempster (5-5) went six innings for his fourth win in his last five decisions this season and first win in five decisions as a visitor at the ballpark he used to call home.

The right-hander, who pitched for the Reds in 2002 and 2003, allowed three hits and one run with one walk and eight strikeouts to break a tie with Kirk McCaskill for the second-most major league wins by a Canadian-born pitcher. Dempster has 107, behind only Hall of Famer and former Cubs ace Ferguson Jenkins (284).

Kerry Wood and Sean Marshall each pitched a scoreless inning before Carlos Marmol pitched the ninth for his 11th save.

Reds starter Bronson Arroyo (4-6) lasted 6 1-3 innings on the sunny, steamy afternoon, giving up eight hits and four runs — three earned — with one walk and three strikeouts as the Reds finished a nine-game homestand with five wins.

The game-time temperature was 90 degrees.

Arroyo’s solid start was the third straight for the Reds, leaving him feeling as if the Reds blew a golden opportunity.

“It’s definitely disappointing,” he said. “The way we’re playing lately, we can’t afford to let close games get away.”

The Reds led 1-0 when Blake DeWitt, bumped up to the third spot in the batting order for the first time this season, reached on Janish’s error with one out in the fourth inning. Pena followed with a 443-foot home run into the right field seats, his ninth of the season.

DeWitt was 6 for 8 in the first two games of the series.

Ramirez hit the next pitch 342 feet into the left field seats for his third homer of the season and 27th of his career against Cincinnati.

The Cubs added a run in the seventh when, with Kosuke Fukudome on third and Pena on first with two outs, Arredondo whirled and faked a throw toward an uncovered first base. First baseman Joey Votto was playing off the bag with Ramirez batting. Arredondo was called for a balk, allowing Fukudome to score and Pena to go to second.

“Somebody’s got to tell him (nobody was covering),” Baker said. “We’re trying to cover the hole there. I’ve seen Ramirez hit too many balls through that hole. You’ve got to communicate.”

The Reds loaded the bases with one out in the first inning, but Dempster coaxed Miguel Cairo — who hit a grand-slam Tuesday night — into a popup before Fred Lewis grounded out to the end the threat.

They took a 1-0 lead in the third. Drew Stubbs led off with a soft liner to center for a single, stole second and moved to third on Brandon Phillips’ groundout to second. Votto struck out, but Jay Bruce lined a 3-2 pitch to right field for an RBI single.

NOTES: Two Cubs on the disabled list, OF Reed Johnson and IF Jeff Baker, are scheduled to start rehab assignments Friday with Triple-A Iowa, and OF Alfonso Soriano is due to join them Sunday, manager Mike Quade said. … Reds RHP Jared Burton, who hasn’t pitched this season while dealing with shoulder inflammation that required surgery, will report to Cincinnati’s Goodyear, Ariz., complex on June 12, manager Dusty Baker said. “It’s like spring training all over again for him,” Baker said. … Reds 3B Scott Rolen missed his fourth consecutive start with a viral infection in his throat and not strep throat, Baker said, quoting a team doctor.