פירוש על קידושין 32:7

The four does not include the death of the master. This can be shown by the end of the mishnah—for if the mishnah had meant to include this, there would be four ways for a female slave to go free.
Thus the mishnah omitted two ways of going free—death of the father and death of the master, and we have, for the time being, resolved the difficulty raised by R. Sheshet on Resh Lakish.