This is a MacBook Pro running macOS High Sierra (10.13.6). The specific version is important.
I'm using the Download page https://www.pjrc.com/teensy/td_download.html
Download from the link "Macintosh OS Software (10.10 - 10.15)". This gives "Teensyduino_MacOS_Catalina.zip". Unpack it to get "Teensyduino.app". Put that into /Applications and the Dock, and run it. It compiles OK, but cannot upload the sketch to my Teensy 4.0. I traced this to the fact that inside the app bundle, Teensy.app is built for a more recent version of Mac OS than I am running (even though my version is within the limits given in the link).
Download from the link "Macintosh OS-X Installer (10.7 - 10.14)". This gives a .dmg containing an installer that modifies an existing Arduino.app to know about Teensy boards. It works initially, including uploading sketch to board, but then starts crashing immediately, before displaying the splash screen. Re-run Arduino.app and it crashes again. And again. And again. Rebooting the laptop does not help. Disconnecting the USB to the board does not help. It seems the app bundle gets corrupted, or some essential data it uses during startup.
There does seem to be a workaround: copy Teensy.app from inside the second into the first (they are at different places in the file tree). Teensyduino.app now works, can upload sketches, and does not crash upon startup.
Since the first download has "Catalina" in its name, I'm guessing that it probably works OK on that version of Mac OS. But as downloaded, NEITHER works for me, on maOS High Sierra (10.13.6).
I'm using the Download page https://www.pjrc.com/teensy/td_download.html
Download from the link "Macintosh OS Software (10.10 - 10.15)". This gives "Teensyduino_MacOS_Catalina.zip". Unpack it to get "Teensyduino.app". Put that into /Applications and the Dock, and run it. It compiles OK, but cannot upload the sketch to my Teensy 4.0. I traced this to the fact that inside the app bundle, Teensy.app is built for a more recent version of Mac OS than I am running (even though my version is within the limits given in the link).
Download from the link "Macintosh OS-X Installer (10.7 - 10.14)". This gives a .dmg containing an installer that modifies an existing Arduino.app to know about Teensy boards. It works initially, including uploading sketch to board, but then starts crashing immediately, before displaying the splash screen. Re-run Arduino.app and it crashes again. And again. And again. Rebooting the laptop does not help. Disconnecting the USB to the board does not help. It seems the app bundle gets corrupted, or some essential data it uses during startup.
There does seem to be a workaround: copy Teensy.app from inside the second into the first (they are at different places in the file tree). Teensyduino.app now works, can upload sketches, and does not crash upon startup.
Since the first download has "Catalina" in its name, I'm guessing that it probably works OK on that version of Mac OS. But as downloaded, NEITHER works for me, on maOS High Sierra (10.13.6).