ISS015-E-23053 (18 Aug. 2007) --- Astronaut Dave Williams, STS-118 mission specialist representing the Canadian Space Agency, participates in the mission's fourth and final session of extravehicular activity (EVA) as construction and maintenance continue on the International Space Station. During the 5-hour spacewalk, Williams and astronaut Clay Anderson (out of frame), Expedition 15 flight engineer, installed the External Wireless Instrumentation System antenna, attached a stand for the shuttle's robotic arm extension boom and retrieved the two Materials International Space Station Experiments (MISSE) to be brought back on the shuttle.