* Nikkei gains 3.9 percent on the week * DeNA soars on strong results, pulls up Gree * Sony firms on So-net purchase * Sharp scrambles back up 9 pct on short squeeze By Sophie Knight TOKYO, Aug 10 (Reuters) - Japan's Nikkei share average fell on Friday as investors lacked fresh incentives to buy, but the index still marked its biggest weekly gain since February following a four-day rally spurred by hopes for more global stimulus.