Dear Jayant, now your Aadhar card is not mandatory for aplying for NEET exam. The last 4 digits are your identity specific. The last 4 digits of your Adhar card possibly wouldnt have been allocated to anyone else. The last 4 digits are enough to prove your authenticity just like the SSN no# in the US. So if you are asked only to enter the last 4 digits, please enter only the last 4 digits.