Two of the crew members were to float outside Thursday night to squirt salmon-colored caulk into the crevices of extra space shuttle thermal tiles that were deliberately damaged for the test. NASA wants to see how well the caulking gun and patching material work, in case they're ever needed for a real repair.
The tools were developed in the wake of the 2003 Columbia disaster. The shuttle was destroyed and all seven astronauts were killed during re-entry because of a hole in the wing.
"Having this in our bag of tricks is really going to be helpful," astronaut Robert Behnken said Wednesday night.
Behnken and Michael Foreman will work on sample tiles that were carried up in Endeavour's payload bay.
The experiment was supposed to be conducted during a shuttle flight last year but was scrapped because of urgent repairs needed for a ripped solar wing at the international space station.
NASA would like the test results before Atlantis blasts off at the end of August on one last repair mission to the Hubble Space Telescope. The astronauts on that mission will not be able to use the space station as a refuge if their shuttle is damaged during launch because they won't be in the same orbit.
Another space shuttle will be on the launch pad ready to fly to the rescue if necessary. Nonetheless, NASA wants the Hubble crew to have as many shuttle repair methods available as possible.
A fifth spacewalk is planned for Saturday, two days before Endeavour undocks from the orbiting complex following a nearly two-week visit.
The shuttle astronauts spent the first half of their mission putting together the space station's new Canadian robot, Dextre, and installing a Japanese storage compartment that will be followed by Japan's enormous Kibo lab in May.
Japan's prime minister, Yasuo Fukuda, called with congratulations Wednesday night and was treated to a televised tour of the space station's new Japanese compartment, courtesy of Japanese astronaut Takao Doi.
Garret Reisman, the space station's newest resident, said he was amazed by the size of the space station when he arrived last week. He noted that when the shuttle was approaching the station, many on the crew were reminded of the 1968 science-fiction film "2001: A Space Odyssey."
"All we needed was `The Blue Danube' playing in the background and it would have been just like the movie," Reisman said in a series of broadcast interviews.