Official Raspberry Pi 4 Case Fan & Heatsink - Adafruit | Mouser
The Raspberry Pi 4 has a fan now - the Case Fan | Jeff Geerling
Amazon.com: GeeekPi Fan Hat for Raspberry Pi 4 Model B, PWM Fan GPIO Expansion Board with 0.91inch OLED Display for Raspberry Pi 4B/3B+/3B/2B : Electronics