Now that more details about the PJRC Teensy 4.0 is out, its confirmed it does not have pin compatibility with the I2S (audio) or SPI busses. The PJRC audio board is also not compatible with the Teensy 4.0. I will be making a small adapter board to covert the TGA Pro socket to accept a Teensy 4.0 and make that available since it's just pin assignment that has changed. Otherwise it's compatible.