What figure of speech is used in this line from "Macbeth" Act IV, Scene 3?" Let us rather, hold fast the mortal sword and like good men Bestride...

I would say that there are two figures of speech used in this quote.  I would say that Shakespeare is using hyperbole and personification.

In this scene, Malcolm and Macduff are talking about what to do next.  Malcolm just wants to sit down and cry, but Macduff speaks the line you cite.  In it, he is saying that instead of sitting down and mourning, they should grab their swords and defend Scotland (their "birthdom").

I think this is hyperbole because the two of them alone cannot really defend Scotland -- Macduff is exaggerating what they can do so as to encourage Malcolm.

I think it is personification because "bestride" means to stand over someone who's on the ground and defend them.  Macduff is saying Scotland is like a wounded comrade lying on the ground and they have to bestride it and defend it.