The Pirates battered Nationals starter John Lannan for four runs in the fourth inning Monday. (Gene J. Puskar/Associated Press)

The Washington Nationals were supposed to have the day off on Monday, but because rain postponed Friday’s series opener against Pittsburgh, the team had to stay an extra night in order to complete the rubber match.

Circumstances then turned even more unpleasant when the Pirates scored all the runs they needed in the fourth inning off starter John Lannan, and Washington failed to hold an early advantage on the way to losing, 4-2, for the fourth time in five games in a rain-delayed game before an announced 12,457 at PNC Park.

The loss dropped the Nationals below .500 and was Lannan’s second in his past three starts. The left-hander was working on normal rest after Friday’s postponement, and although he opened strong, the Pirates sent eight batters to the plate in their decisive fourth in support of starter Paul Maholm’s first victory of the season and first against Washington in eight career starts.

“No excuses. We got off to a good start and kind of flattened out,” Nationals Manager Jim Riggleman said. “Maholm started making some pitches, and we didn’t get it done.”

The Nationals (10-11) had the upper hand early when they struck in the first inning before a band of heavy showers began pelting the park, prompting fans to flee for cover. The game went on, though, and Washington had Maholm on his heels, starting with shortstop Ian Desmond’s single to center while batting from the No. 2 spot.

Right fielder Jayson Werth followed with a double that sailed over the head of right fielder Matt Diaz and bounced off the scoreboard for his first hit of the season against a left-hander. Desmond, meantime, stopped at third. First baseman Adam LaRoche then laced Maholm’s first-pitch slider to center, scoring both runners.

The Nationals threatened again in the third when with two outs, Werth singled past diving second baseman Neil Walker. LaRoche walked to reach base for a second straight time, but Michael Morse left both runners stranded by striking out on a change-up. Morse, however, began the Pirates’ half of the inning with a catch at the warning track, turning potential extra bases off the bat of Chris Snyder into the first out.

The rest of the third was uneventful for Lannan, who got Ronny Cedeno to ground out and struck out Maholm looking during a 10-pitch at-bat. At that point, Lannan had faced the minimum through three innings, including benefiting from a first-inning double play that erased the only hit he had permitted.

In the fourth, Lannan issued a one-out walk to Jose Tabata and a two-out single by Walker. That hit wound up in Werth’s glove and shouldn’t have caused any more damage, but catcher Wilson Ramos was unable to handle the one-bounce throw to the left of the plate that was scored as a throwing error on Werth. The ball rolled to just in front of the Pirates dugout, where Lannan picked it up, but not before Tabata came home and Walker ended up on third.

That’s when, one pitch into an at-bat against Steve Pearce, the most soaking rain of the night forced a stoppage in play that lasted 21 minutes. When play resumed, Lannan walked Pearce to put runners on the corners, and Brandon Wood drove a change-up to right-center field to clear the bases to give Pittsburgh (10-12) the lead for good, 3-2.

Wood came home from second one batter later on Snyder’s sharp single to left, and Lannan finally got out of the inning by securing Cedeno’s hard comebacker and throwing to LaRoche.

“Doesn’t matter the circumstance,” Lannan said of the delay. “You’ve just got to make pitches, and I was just up in the zone, and it hurt.”

Lannan (52 / 3 innings, five hits, four runs) was unable to get out of the sixth, though a questionable call and the defense behind him were partly to blame. After getting Diaz to fly out to right to lead off the inning, Walker singled to left on a sizzling grounder between third and short. Lannan soon after threw to first with Walker leaning the wrong way, and LaRoche appeared to tag out the runner well before he got back to the base.

First base umpire Alan Porter didn’t agree, ruling Walker safe despite protests from LaRoche and Riggleman, who came onto the field to dispute the call. One batter later, Wood grounded to short but was safe when Desmond lost the ball while trying to make the throw to first. Lannan exited after that batter, and reliever Todd Coffey fanned Snyder to end the frame.

The Nationals threatened to spoil the win for Maholm (seven innings, four hits, two runs, eight strikeouts) in the ninth by putting runners on first and third with two out, but closer Joel Hanrahan got pinch hitter Rick Ankiel to strike out looking for his sixth save.

“You’ve just got to keep grinding and keep battling,” said Jerry Hairston, who started in center to give Ankiel some rest. “We feel we’re a good team. If we keep working hard, things will turn around.”