It ends up that my DAC has both a USB-B and and a USB-A on it. The USB-A is apparently only to connect iOS devices. When I take a USB-A cable from the RPi4 to the USB-B input on the DAC, it just shows up…

$ lsusb
Bus 001 Device 006: ID 29c5:0022 Peachtree Audio Peachtree preDAC

And now I’m on to my next problem. Everything I try to play says “Couldn’t start playback”. My other plexamp clients don’t have this problem. Strangely, I can run app.plex.tv in a browser on the RPi4 and play the same music library that Plexamp refuses to play. I’m pretty close.