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Hello.  
In order to use GPIO you can install OPi.GPIO Libraries. If you are using debian or ubuntu images for the OPi4/4B the instructions are very straightforward: 
1.- cd /usr/local/sbin/OPi.GPIO/
 
  Copy code 
2. 
3. 
4.- sudo python test_gpio.py 11
 
  Copy code (to test pin 11, blinking a led) 
 
Then, you can connect a fan through a transistor for pulling current from power pins, or attatch to 5.0V pin and GND (Pin 4 and 6 respectively) |   
 
 
 
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