TOKYO, April 5 (Reuters) - Japan's Nikkei average fell to a four-week low in early trade on Thursday after a poor Spanish debt auction and fading hopes of further U.S. monetary stimulus, extending the previous session's 2.3 percent slide, its worst day in five months. The Nikkei shed 0.7 percent to 9,747.80, on track for its third straight losing day. The broader Topix index was down 1.1 percent at 826.65.