Latest update...
My first attempt at MicroDexed with T4 (using PT8211) worked great, except for occasional glitches in the audio, which I suspect were caused by my sloppy wiring between T4 and PT8211; there are warnings about avoiding wires for this...
So, I decided to rebuild, using a Teensy Audio Board mounted directly atop the T4. (Yes, I got the Rev.D).
Changed config.h to use Teensy Audio, got it working using USB MIDI.
Latency is much worse, unplayable.
In the MicroDexed code (config.h):
// ATTENTION! For better latency you have to redefine AUDIO_BLOCK_SAMPLES from
// 128 to 64 in <ARDUINO-IDE-DIR>/cores/teensy3/AudioStream.h
Note: I didn't do that using PT8211, and latency was acceptable (using DIN MIDI or USB MIDI).