I am running into the same issue discussed here:
http://slf4j.42922.n3.nabble.com/Sigar-and-log4j-over-slf4j-td2165576.html

Sigar reports a stack trace like:

java.lang.NoSuchMethodError: org.apache.log4j.Logger.getAllAppenders()Ljava/util/Enumeration;
at org.hyperic.sigar.SigarLog.isLogConfigured(SigarLog.java:40)
at org.hyperic.sigar.SigarLog.getLogger(SigarLog.java:49)
at org.hyperic.sigar.SigarLog.getLogger(SigarLog.java:44)
at org.hyperic.sigar.SigarLog.debug(SigarLog.java:60)
at org.hyperic.sigar.Sigar.<clinit>(Sigar.java:108)

And it was traced to the absence of getAllAppenders() in log4j-over-slf4j. Adding the missing method addresses the Sigar issue...

public Enumeration getAllAppenders() {
return new Enumeration() {
public Object nextElement()

{ return null; }

public boolean hasMoreElements()

{ return false; }

};
}

I'm unclear if the above method should be properly implemented, or if the mock stub is sufficient.