The revision 4 features the new speaker! It supports both Cherry MX and Matias/ALPS switch footprints for both the MIT and Grid layouts. All components (USB Mini port, diodes, ATMEGA32U4, 470 ohm resistors for keyswitch LEDs, speaker, etc) are pre-soldered and it's ready for keyswitches and LEDs.
The PCB features a reset button for quick programming (milled bottoms have a hole at the precise location of this). The standard layout already flashed to unit.
There are 5 mounting slots that are compatible with the milled bottom. Other mounting holes are for plateless mounting on custom cases. Then remaining pins on microcontroller are pinned-out for easy modding.