I put mine in shared/classes/alfresco/extension.
I use dev-log4j.properties. That's just the way I've always done it.
It looks like you can use whatever file name you want e.g.
custom-log4j.properties
Here's the relevant configuration:
<value>classpath*:alfresco/module/*/log4j.properties</value>
<!– Enterprise extensions –>
<value>classpath*:alfresco/enterprise/*-log4j.properties</value>
<!– Other installed extensions –>
<value>classpath*:alfresco/extension/*-log4j.properties</value>
<!– private developer overrides –>
<value>classpath*:alfresco/extension/dev-log4j.properties</value>