Yan,
Single log out is on by default (this is the case in 5.x.), and the default conditions are: LOGOUT_URL is the one that was used during login, back channel is default LOGOUT_TYPE.
These log configs may help (they may be different in 4.x):
<!-- DEBUG service status and logout process and a lot of details -->
<AsyncLogger name="org.apereo.cas.logout" level="info" />
<!-- INFO Performing logout operations for [TGT-...]
[number] logout requests were processed
DEBUG ST, principal and URL -->
<AsyncLogger name="org.apereo.cas.logout.DefaultLogoutManager" level="info">
<Filters>
<ThresholdFilter level="INFO" onMatch="ACCEPT" onMismatch="NEUTRAL" />
<RegexFilter regex="Captured logout request.*" onMismatch="DENY" />
</Filters>
</AsyncLogger>
<!-- DEBUG Logout request will be sent to but does not print anything when login was through SAML 1.1 -->
<AsyncLogger name="org.apereo.cas.logout.DefaultSingleLogoutServiceLogoutUrlBuilder" level="warn" />
<!-- DEBUG preparing, processing and logout with URL and ST -->
<AsyncLogger name="org.apereo.cas.logout.DefaultSingleLogoutServiceMessageHandler" level="debug" />
<!-- DEBUG SAML logout payload -->
<AsyncLogger name="org.apereo.cas.logout.SamlCompliantLogoutMessageCreator" level="warn" />
Ray