Bangladeshis are like illegal mexicans in America , everybody knows they are there but you can not do anything . If you try to send illegal mexicans back to mexico by force then there will be great uproar , hue and cry from human rights groups, NGO , liberal , left communists , democrats , these people are potential prospective groups who will vote and support to democrats .If they illegally cross the border then the responsibility of arresting them lies with the BSF.
4 crores? What are they doing in India? Even the USA and Saudi Arabia don't have 4 crores illegal Bangladeshis in their respective countries.