After the witches show Macbeth the three partially reassuring apparitions, he asks what is apparently foremost in his mind.

Yet my heart
Throbs to know one thing: tell me, if your art
Can tell so much, shall Banquo's issue ever
Reign in this kingdom?   (4.1)

They reluctantly show him a montage or panorama described in the stage directions as:

A show of eight Kings, and Banquo last with a glass in his hand.

Macbeth is horrified because his worst fears are confirmed. He has murdered a king whom he revered and who had treated him with the utmost kindness, and he has sold his soul to the devil, all for the benefit of Banquo and his descendants. As Macbeth says in a soliloquy before he has Banquo ambushed:

then prophet-like
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding. If't be so,
For Banquo's issue have I filed my mind,
For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man,
To make them kings, the seed of Banquo kings!  (3.1)

After Macbeth sees the fourth apparition he is never the same man again. He is disillusioned and depressed. His life seems pointless. He has to force himself to keep up appearances as a ruler, but the title seems meaningless to him. He is on a downhill path after this revelation of the future he feared and anticipated. He has been beaten by Banquo in spite of the fact that he had Banquo murdered. Macbeth was driven by his and his wife's powerful ambition, but he has seen that it was nothing but a vain illusion. They have achieved nothing. His wife says the same thing earlier. 

Nought's had, all's spent,
Where our desire is got without content.
’Tis safer to be that which we destroy
Than by destruction dwell in doubtful joy.  (3.2)

In Act IV, scene 1 of Macbeth our title character has returned of his own accord to see the witches.  In this meeting, he demands that the witches reveal his future.  The witches raise three apparitions from their boiling cauldron.  These first apparitions serve to boost Macbeth's confidence.  The first, an armed head, provides a warning to "beware of Macduff".  The second, a bloody child, states that no man born of a woman will be able to harm Macbeth.  The final apparition tells him that he will not be defeated until the trees at Birnam Wood move to the castle.  Macbeth hearing this becomes confident that his crown is secure until the fourth vision is revealed. 

This fourth vision reveals a line of kings that do not at all resemble him.  The vision shows that Banquo will be the father of the royal lineage.  Macbeth had hoped that by killing Banquo and putting the hit out on his son, Fleance he could change...