| Quote #4
When flowing cups pass swiftly round
The "flowing cups" seem like a metaphor for freedom. First off, they flow, and they're also not diluted, not "confined" or "inhabited" by any sort water (the "allaying Thames").
| Quote #5
When, like committed linnets, I
These lines mimic the speaker's confinement. Lots of letters are repeated (notice the alliteration of the words that begin with s and m), which suggests that—even on a formal level—the speaker's choice of letters is "committed" or "imprisoned" as well.
| Quote #6
Stone walls do not a prison make,
If "iron" and "stone" don't make "prisons," what does? The speaker suggests that the only real prisons out there are ones people make themselves. The human mind is more powerful than strong building materials like iron and stone.