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.
The device is as wide as the Raspberry Pi 4. The HAT uses its own 32- bit processor, receiving input from the I2C interface. A step-up power supply is necessary to convert the 5V output from the Pi to 12V.
According to the dev team, it's possible to stack too many HATs, further compounding the heat issue. If necessary, makers can splice multiple Smart Fan HATs into a HAT stack for maximum cooling efficiency.
The team recently met their funding goal on Kickstarter; you can read more about the HAT and what to expect on the project page. Be sure to check out our list of Best Raspberry Pi Projects for more cool creations from the maker community.