you can use
java script to submit the form where you can attach a parameter with the action and retrieve it in the servlet
OR
you can have four different servlet-mapping for the same servlet and use java script to submit to each of those mappings depending on the button being clicked.
In this way you can use getRequestURI() on the request object to know from which url(specified in the deployment descriptor) the request came from and call the appropriate method in your servlet