Stanza 4 Summary

Lines 13-16

But most thro' midnight streets I hear
How the youthful Harlot's curse
Blasts the new-born Infant's tear,
And blights with plagues the Marriage hearse.

• The poem's final stanza has arrived. After that whole business about "mind-forg'd manacles," these are the most famous lines in the poem.
• The speaker hears lots of things, but "most" of all he hears a youthful harlot at midnight—not just any old harlot (prostitute), but one who is young (sigh) and cursing a "new-born Infant's tear."
• Okay, so there's a foul-mouthed prostitute and a "new-born Infant." Is it her child? Maybe, but we don't know for sure. 
• But it doesn't really matter. The point the speaker is making is that babies are born into a world where young women have become prostitutes (harlots), and their tears (babies cry a lot) get cursed at instead of soothed. ("Blasts" here means something like "attacked" or "assaulted," but in a very metaphorical way. It's like saying, "I went out in the street and my ears were blasted by that guy next door's loud lawnmower.")
• In addition to this whole business about children being born into a corrupt, dirty world—cursing, harlots, blasting—there's something else going on. This same harlot-curse, which "blasts" the baby's tear, also "blights with plagues the Marriage hearse."
• Again, like that stuff earlier with the blood and the sighs, this is some really gnarly metaphorical stuff. 
• The words "blight" and "plague" are similar. They both refer to disease—a plague is, well, a plague, whereas the noun "blight" describes a kind of barrenness or infertility usually brought on by drought or disease.
• But "blight" here is a verb, so we'll take it to mean something like "tarnishes," even "mars" or "destroys." 
• Basically, then, the harlot's curse, which is probably a symbol for her terrible life experiences (much like the soldier's sigh is for his), totally ruins the "marriage Hearse." The curse—the fact that there even is a youthful harlot in existence—completely destroys the institution of marriage. It "plagues" it, so to speak. 
• This is why the speaker uses the semi-oxymoronic phrase "marriage Hearse." We associate marriage with children, life, union.
• A hearse, obviously, symbolizes death. Marriage is a "hearse" because, well, unmarried harlots are running around, babies seem to have no mothers (who is the mother of this baby again?), and there are no fathers to be found.
• Marriage has been plagued, we might say, both figuratively and perhaps even literally. How? Well, "plague" may possibly be a reference to venereal disease, which definitely existed in Blake's day. The marriage hearse may be blighted, potentially, by the transmission of whatever diseases the harlot's profession has given her.
• The harlot, in other words, engages in prostitution, which gives her some kind of sexual "plague," which she brings to her marriage (as well as the marriages of her clients).
• But hey, marriage is already a "hearse" anyway (an institution of death)—at least according to this speaker—so this just adds insult to injury.
• So, in the speaker's "London," life is not a bowl of cuddly babies and happily-ever-afters. Instead, it's disease, suffering, and misery. Bad times.