Is it possible to use a J-Link (ie. JTAG) to program the new Teensy 4.0? There appears to be a heavy-duty hack to the Teensy 3.6 board which includes removing a chip to provide for JTAG debugging. Is such a hack required or even possible on the new board?
Unfortunately we have no idea, especially since the Teensy 4.0 just became available today. (Aug 7th 2019)
You might try asking on the [PJRC forum?](https://forum.pjrc.com/)
After checking around a few places on the Internet including the Teensy forum and NXP package diagrams, it appears that the answer is nope, no JTAG. Arduino only. Oh well - I’m not sure what the use for the board is anyway.