Describe the effect of the old man's final words in Act 2, scene 4 of Macbeth.

The old man suggests that Macduff makes the best of a bad situation, as he will do when he confronts Macbeth with Malcolm.

At the end of Act 2, Scene 4, the Old Man suggests to Macduff that he will be able to make the most of the tragic situation.  He seems to have faith that Macduff will be able to turn things around.  Duncan has been killed, but instead of fretting or mourning, Macduff will join Malcolm, where he can fight Macbeth alongside Scotland’s rightful leader.

The old man’s last words in this scene are prophetic.

God's benison go with you 

and with those

That would make good of bad

and friends of foes! (Act 2, Scene 4)

It may seem odd to have some random old man standing around the castle, but the old man seems to serve as a harbinger and commentator of events.  Here he gives a blessing to Ross and Macduff.  Ross is Macbeth’s cousin, and he continues alongside Macduff into the final battle with Macbeth.

Macbeth’s ability to make the most of a bad situation is clear throughout the play.  When his family is killed, he is devastated.  However, he does not let this stop him from his duty or mess with his head.  Rather than being blinded by rage, he is able to face off against Macbeth and take his head off.

I have no words:

My voice is in my sword, thou bloodier villain

Than terms can give thee out! (Act 5, Scene 8)

It is because he is able to keep his head in his exchange with Macbeth that he can get him off guard and off balance.  Macbeth thought he was invincible, but feared Macduff because of the witches’ prophecies.  When Macbeth told Macduff that he could not be harmed, Macduff told him he really could.  Macbeth was not born the usual way, and that fact demoralized and frightened Macbeth enough to distract him so that Macduff could kill him.

Shakespeare often uses no name characters to say important lines.  Their words can point to future events or the importance of certain characters.  They can also comment on what is happening or predict what is coming.  The old man serves this role in the play.