TOKYO, April 5 (Reuters) - Japan's Nikkei share average fell to a four-week low on Thursday, stung by revived concerns about Europe's funding difficulties after a weak Spanish debt auction and fading hopes for additional stimulus from the U.S. Federal Reserve. The benchmark Nikkei slipped 52.38 points to 9,767.61, logging its third day of losses, while the broader Topix index fell 0.3 percent to 832.57.