Reset

All 3.x and 4.x Teensies can do a reset in software:

SCB_AIRCR = 0x05FA0004;
asm volatile ("dsb");

Similar to reset, but starts the bootlader:

asm volatile ("BKPT 254");

The Teensy 4 can be switched off in software, too.
!!Attention!! It switches off immedately, so adding this to setup() is a bad Idea.

  SNVS_LPCR |= SNVS_LPCR_TOP; //Switch off now
  asm volatile ("dsb");

For using the Power-Pin on the Teensy 4 you can use this library:

https://github.com/TeensyUser/doc/wiki/Teensy-Libraries#teensy-4041-power-button-support