I could not find directions for this online, so after I figured it out, I figured I’d post it here so it’s somewhere when someone else runs into this issue.
I don’t have a power button on my S4. It was defective and bootlooping, so I had it removed, but removing it removed a solder pad too, so I can’t put a power button back. Adb to the rescue!
Naturally, you’ll have to have debugging enabled, ADB installed, a working USB port, etc., but there are directions for that elsewhere.
I could get into recovery by means of:
ADB reboot recovery
But once there, selecting is accomplished with the power button, at least with the recovery I had. D’oh!
After not finding any directions and trying a bunch of things, I finally just tried the following, which worked:
ADB reboot download
Done. Booted right into download mode.