Most probably for replacement. Teensy 3.2 and Teensy 4.0 are not fully pin-compatible, e.g serial ports, CAN bus ports, etc. And don't forget the higher powerconsumption that comes with a Teensy 4.0. Besides that, a Teensy 4.0 puts out higher frequency emissions (which may or may not be a problem for a specific application).but there must be a reason when you can get a 4.0 for a fraction of the price?