Hi Nisha,
In ABAP, we can create alpha numeric number. In order to do that, it is required to create new FM/Class for generating the alpha numeric number . Please find the link below.
Alpha Numeric Number Range (0000 to ZZZZ) - Code Gallery - SCN Wiki
Note:
This code will generate new number when the user passes the current one, it is required to maintain the current number range in some table like TVARV.
Example:
Input - AAAA,
Output - AAAB.
Regards
Rajkumar Narasimman