Astronauts wrap up outside work on space station

Astronaut Jerry Ross works from the end of the space station's robot arm, extending from the right side of this picture. The station's giant, gold solar array is behind Ross.

(CNN) -- Atlantis astronauts Jerry Ross and Lee Morin concluded the shuttle crew's fourth and final spacewalk on Tuesday, with the orbiter nearing the end of its 11-day construction mission to the international space station.

Tuesday's spacewalk lasted more than six hours. Tasks include installation of a 14-foot beam extending from the space station's U.S.-built Quest air lock to help future spacewalkers maneuver around the station more efficiently.

Ross and Morin also brightened up the space station. They hung halogen work lights on the outside of the station's Unity module and the Destiny science lab. The 40-watt lamps will shine a 9- by 7-foot elliptical beam of light 20 feet away.

Early Tuesday, a misconfigured valve in the Quest air lock caused a brief delay during spacewalk preparations. As Ross and Morin got set to go out, a swooshing sound was heard in the air lock.

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"Houston, I think the hatch just burped again," said space station resident Dan Bursch, who was helping the spacewalkers get ready. "I heard a big rush of air. I'm not quite sure where it came from."

NASA spokesman Kyle Herring said when crew members closed the hatch between the station and the air lock, the air lock didn't depressurize as it should have.

"They had a valve misconfigured between the air lock and space station," Herring said.

Once they reset the valve, the hatch seated properly, and depressurization continued normally. Herring said it put the crew about 15 minutes behind schedule, but the crew made up the time. Herring said the valve problem didn't pose any dangers.

Other exterior chores on Tuesday included attaching handrails and shock absorbers to the new space station railcar. The railcar was installed during the earlier spacewalks on this mission.

Space station Alpha

A test run was done late Monday, but there was a snag. The railcar moved, but apparently floated slightly off the track and caused a magnetic sensor to lose contact. Eventually, the crew completed the test.

The railcar will be used to allow the space station's 58-foot Canadian-built robot arm to move from one end of the station to the other.

Atlantis is scheduled to undock from the space station Wednesday afternoon and land Friday at Kennedy Space Center in Florida.