yes you can
i think you should following this steps
1-At first you must register your users with specific ID on your Online Database
2-build your server by b4j
3-connect to server from b4a
4-if connected then you can send your user id to server
5-if ID recieved to server side then you can select and update your online data base 4 Relevant ID
now any other user on your server can recieve updated data
i think this steps are correct but
@Erel can said this is correct and is best way or not
regards