Phase IV will cover about 440 km by 2021
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/delhi-metro-phase-iii-add-103-km-existing-network
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi-metro-rail-corporation-dmrc
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/ministry-urban-development
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/public-transport
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/urbanisation
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/london
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environment
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/transport