T4 to T3 Adapter - pixelmatix/SmartMatrix GitHub Wiki
An adapter has been designed to take a Teensy 4.0 or Teensy 4.1 and directly connect to the SmartLED Shield (V4) for Teensy 3. It can be custom ordered from OSH Park, although some assembly is required.
See this forum post for details: https://community.pixelmatix.com/t/teensy-4-0-released/498/32
Note the adapter differs from SmartLED Shield for Teensy 4 (V5) in the default pin used for CLK
Todo: describe the jumper configuration options