Leibniz's Law

Which of the following arguments involves a valid employment of Leibniz's Law? Why?

1. Oedipus married Jocesta. Jocesta is Oedipus's mother. So Oedipus married his mother.
2. Jones might have come first in the race. Jones was the person who came second. So the person who came second might have come first.
3. $\sqrt{{2}}$ is not rational. 1.4 is rational, so $\sqrt{{2}}$ is not 1.4.
4. I can conceive that my body does not exist. I cannot conceive that I do not exist. So I am not my body.

Readings

(Hodges, 2001, §§26-29)

(Sainsbury, 2001, ch. x)