I've been tickled pink at being recognized as potentially useful beta tester
-- Thanks!
But as of now, I have my hands full
(with stuff involving 4.0).
I will be getting a Teensy 4.1 later though, when they become widely available; hopefully Mandu.fi will order/get a batch then.
In particular, the larger number of pins on 4.1, and the Ethernet interface, makes it a perfect choice for finally implementing my curved velocity controlled Cartesian robot (3D printer or CNC). (The math essentially implements an iterator that walks the intersections between a 2D cubic curve and integer coordinates, producing the exact correct step list tracing the curve; with a velocity controller tied into stepper feedback (digital Hall angle sensors or AS5048 sensor chips at the stepper motor butt ends) to recover lost steps, and generating the actual pulse trains. I don't know yet of any slicer that produces curves instead of lines, or can use thinner layers on the outside compared to inner structural lattices, so this is a rather low-burn, long-term project... I really just found the math to be much, much fun to work out some years ago.)
As of now, I'm investigating different methods of implementing "better" Teensy 4.0 CDC ACM, specifically for robust USB serial/tty on Linux with higher transfer rates. The reason is I believe Teensy to be most suitable for implementation of custom scientific sensors (stuff from 24-bit ADCs like TI ADS1256 to full data acquisition chips like 8-channel 16-bit AD7606, to MAX31865 PT100/PT1000 temperature sensors) -- some colleagues needing both temperature sensors and 0-10V voltage sensors, connected to a Linux SBC --, but considering average scientist programming skill level, being able to use the termios/tty/USB Serial interface from pure Python3 (instead of USB bulk transfers) would be much more practical. As of now, the CDC ACM/USB Serial on Teensy 4.0 is likely to deadlock with large transfer rates.