I assume you are referring to the articles explaining how to configure log4j logging to log to the BAM server for storage in Cassandra. They are applicable to the IS server as well since the IS is, just as all WSO2 products, based on the same base product called 'Carbon'.
If you specifically want to log only the login/logout events and not any additional logging you could adjust the appender in such a way that only the following class will log to that appender (as shown here: log4j: Log output of a specific class to a specific appender):
log4j.logger.org.wso2.carbon.core.services.util.CarbonAuthenticationUtil=INFO, LOGEVENT
log4j.additivity.org.wso2.carbon.core.services.util.CarbonAuthenticationUtil=false