* Q4 net profit down 1.8 pct year on year* Sixth straight quarterly profit drop* Trims H2 2014 dividend to 0.75 riyals/share* Shares down 2.7 pct in opening minutes of trading(Recasts, adds analyst comment, context)DUBAI, Jan 15 (Reuters) - Al Rajhi Bank 1120.SE , Saudi Arabia&aposs second-largest listed lender, posted a 1.8 percent drop in its fourth-quarter net profit, marking the sixth straight