Are 4 witnesses required for Slaves?

Sahih Muslim Book 17, Number 4221 > Abu Huraira reported that Allah's Messenger (may peace be upon him) was asked about the slave-woman who committed adultery and was not protected (married). He said: If she commits adultery, then flog her and if she commits adultery again, then flog her and then sell her even for a rope. Are 4 witnesses required for slaves?