In Episode 4 of Oedipus Rex, what do you make of the following coincidences concerning the herdsman?
The herdsman: (a) saved infant Oedipus from death (b) was the lone survivor of the attack on Laius and also the sole witness to the attack, & (c) will provide testimony that will destroy Oedipus?
I think you are slightly mistaken in your focus on the role of the Herdsman. He only provides information concerning the true parentage and origins of Oedipus, and was not the sole survivor of the attack on Laius and the sole witness.
The role of the herdsman or shepherd obvioiusly is the missing piece of the jigsaw that Oedipus has been hunting for throughout the entire play. He is able to give voice to the fears and suspicions that Oedipus has had throughout the play. It is the herdsman himself, who, after being threatened, says he is on the "bring of dreadful speech" that Oedipus has insisted he delivers, even though Oedipus recognises that he is on the brink of "dreadful hearing." Note what the herdsman tells Oedipus and how he gives witness that will lead to the downfall of his king:
I pitied the baby, my King,
And I thought that this man would take him far away
To his own country.
He saved him--but for what a fate!
For if you are what this man says you are,
No man living is more wretched than Oedipus.
The herdsman presents therefore the inevitable culmination of the self-discovery of Oedipus as he comes to realise his own identity and what he has done. He has already worked out that it was he who killed Laius, his father, and in this scene receives the final infallible proof that reveals to him that he has killed his father and married his mother.