Nokia Universal Portable USB Charger DC-18 - DC-18 is fully charged and the micro-USB has been unplugged from my phone. 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. 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.