Here is a first beta test for Teensyduino 1.52.
Edit: links removed, please use 1.52-beta5.
Changes since Teensyduino 1.51:
USB Audio support for Teensy 4.0
OctoWS2811 ported to Teensy 4.0
WS2812Serial support for RGBW SK6812 LEDs
FreqMeasureMulti ported to Teensy 4.0 (KurtE)
Dual & Triple USB Serial (Geert Uytterhoeven)
Audio support for 4,6,8 channel I2S on Teensy 4.0
Audio support for SPDIF input on Teensy 4.0 (FrankB)
Audio DAC support for Teensy LC (Ben Kurtz)
Audio PWM output for Teensy 4.0 (FrankB)
Fix Serial.print() conflict with USB Touchscreen
Fix contact count on USB Touchscreen (purple-ben)
Defined FLASHMEM on Teensy 3.x, for compatibility with Teensy 4 code (FrankB)
Fix cache handling in I2S audio input on Teensy 4.0 (Frank B)
Fix numerous iMXRT register field defines
MTP USB descriptors on Teensy 4.0 - actual MTP support still WIP
Libraries updated: FlexCAN_t4, ST7735_t3, RA8875
Edit: links removed, please use 1.52-beta5.
Changes since Teensyduino 1.51:
USB Audio support for Teensy 4.0
OctoWS2811 ported to Teensy 4.0
WS2812Serial support for RGBW SK6812 LEDs
FreqMeasureMulti ported to Teensy 4.0 (KurtE)
Dual & Triple USB Serial (Geert Uytterhoeven)
Audio support for 4,6,8 channel I2S on Teensy 4.0
Audio support for SPDIF input on Teensy 4.0 (FrankB)
Audio DAC support for Teensy LC (Ben Kurtz)
Audio PWM output for Teensy 4.0 (FrankB)
Fix Serial.print() conflict with USB Touchscreen
Fix contact count on USB Touchscreen (purple-ben)
Defined FLASHMEM on Teensy 3.x, for compatibility with Teensy 4 code (FrankB)
Fix cache handling in I2S audio input on Teensy 4.0 (Frank B)
Fix numerous iMXRT register field defines
MTP USB descriptors on Teensy 4.0 - actual MTP support still WIP
Libraries updated: FlexCAN_t4, ST7735_t3, RA8875