| Quote #4
Yet thou triumph'st, and say'st that thou
OK, so she doesn't buy the marriage argument. In fact, she literally squishes the whole little marriage-inside-the-flea. But the speaker comes right back at her, saying that if she doesn't feel weaker after having killed the flea, she won't feel weaker after having sex with him. It's another absurd argument, to be sure.
| Quote #5
'Tis true; then learn how false fears be;
After a long digression, the poem comes full circle back to the idea that having a tryst would not be such a big deal. Once again, the speaker attempts to use the flea's small size to convince the woman that her "honour" (another word for reputation) would not suffer if they did the deed.