The Nikkei stock index snapped a four-day winning streak Monday, as investors moved to lock in gains amid concern the market is becoming overheated after a recent rally led to all-time highs late last week. The 225-issue Nikkei Stock Average ended down 474.31 points, or 1.16 percent, from Friday at 40,414.12. The broader Topix index finished 35.58 points, or 1.26 percent, lower at 2,777.64. On the top-tier Prime Market, decliners were led by precision instrument, service and real estate shares. The U.S. dollar fell slightly in the lower 151 yen range in Tokyo on wariness over possible yen-buyi…