If your Raspberry Pi 4 project relies on a HAT, you may want to throw in this stackable Smart Fan HAT by Sequent Microsystems. It cools much more than a single heatsink and has full GPIO access.
There are already fan HATs available for the Pi 4, but the biggest draw to this one is the fully exposed access to GPIO pins. Because the full set of pins is exposed on top, users can stack several HATs together.









