What is the fourth apparition in Macbeth?

After the Weird Sisters show Macbeth the first three apparitions—the helmeted and disembodied head, the bloody child, and the child crowned who holds a tree in his hand—he asks to know if "Banquo's issue [will] ever / Reign in this kingdom" (4.1.116-117). The Weird Sisters caution him that he should not try to learn anything else from the spirits, but Macbeth curses them if they remain unwilling to answer his question. The witches then show him a spectral line of eight kings that all look like Banquo, and the eighth king is holding a mirror in his hand that reflects the line. Macbeth interprets the reflection of the line of eight in the mirror as "show[ing] [him] many more," as though the line will go on and on (4.1.135). The eighth king is followed by the ghost of Banquo. The ghost points at the line and smiles as if to indicate that they are "his" own (4.1.139). Therefore, Macbeth is given to understand that Banquo's issue will most certainly reign in this kingdom and for quite a while.