My phone and DC-18 have both been fully charged. Why does the 4th indicator light still blink to show ongoing charging, although charging has stopped?

DC-18 is most likely fully charged. In this case, the 4th indicator light indicates that DC-18 is ready to charge. Unplug the phone, and tuck in the micro-USB connector to put DC-18 into power save mode. Then the indicator lights switch off to show that charging is complete.