The Indian Foundation of Transport Research and Training (IFTRT) has pointed out that a gazette notification of 2012 by the ministry of road transport and highways made it mandatory for any vehicle
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/bs-iv-norms-being-violated-years-now-says-iftrt
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/times-india-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/ministry-shipping-road-transport-and-highways
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/bharat-stage-iv
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution