1. go to any recognise Airtel retail outlet to
purchase the Airtel 4G
2. Get a 4G enable phone
3. insert your 4G sim card into your 4G  phone
4. Once your phone show the 4G LTE signal
you will receive your 4GB
Data at once.
you can now start browsing for free.

You can still get the 4GB if you buy another AIRTEL 4GB And  insert
into your 4GB phone.

to check your data balance dial *140-#


note:
The 4GB data can only work on 4G enable
smart phone
it is only Valid for,one Month

to continue getting the free 4GB you have to keep buying new airtel sim card.