Four fleet does at a gold valley the famished arrow sang before.
Wowza—now we get some action going on. An arrow chasing after the deer? That's some pretty serious stuff for a poem that was all about naming colors and petting pretty deer in the meadows for the first fifteen lines. (Also, it's important to note that "does" here means four female deer—as in "doe, a deer, a female deer." It's not meant to be read as "duz." That'd make no sense.)
Line 20 is an almost perfect replica of line 10—except now, instead of a bugle singing "before" the deer, an arrow is singing. The arrow is even personified—it's described as "famished," which makes it sound like the arrow is the one that's hungry and hunting, not the person who launched the arrow.
Once again, inversion is what makes these lines so fascinating to read. In order to understand the action of the poem (an arrow is chasing after some deer) you have to read backwards from line 20 to line 19. It's almost as if Cummings is reversing the motion of the chase by forcing us to undo our reading (or at least to backtrack) in order to get a sense of forward motion. Couple that with the repetition (and oh-so-slight changes) between lines 10 and 20, and you get a weird sense of stasis and motion. So, if you're feeling slightly dizzy, you're in good company, Shmoopers.