Fact is that BS-IV vehicles, particularly diesel trucks, are much less polluting than BS-III ones
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/bs-iv-fuss-why-i-ask
[2] http://admin.indiaenvironmentportal.org.in/category/author/sunita-narain
[3] http://admin.indiaenvironmentportal.org.in/category/newspaper/business-standard-new-delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/bharat-stage-iv
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/sunita-narain
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/auto-industry
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/auto-technology