Because of the Servant’s work (Isaiah 53), Zion is restored to her husband (54:4-5). That restoration is marked by the outbreak of song (54:1-3) but also by a series of negations.
Verse 4 begins with three clauses that all begin with the Hebrew particle lo, “not”: Do not fear, you will not be ashamed, you will not be confounded. It continues with parallel clauses that promise deletion of memory: “the same of your youth you will forget” and “the reproach of your widowhood you will not remember.” Isaiah refers to two periods of Israel’s history: She was shamed by her early infidelities, she was reproached because of her Husband’s departure/death. Both of these she will forget.
The waters that Yahweh promises (55:1) might as well be from the river of Lethe. In the joy of reunion with her Husband, Zion forgets all the prior shame and pain of her adulteries and her abandonment.