4-Wired Fan Control (PWM)

I simply want to control a 4-wired fan (or maybe several) with an Arduino board. There is some information out there. Many projects that use a temp sensor, but never the most simple thing: control the speed of the fan.

I found useful code in this thead: https://forum.arduino.cc/index.php?topic=155089.msg1175321#msg1175321

int pwmPin     = 3; // digital PWM pin 9
int pwmVal     = 1; // The PWM Value
unsigned long time;
unsigned int rpm;
String stringRPM;
void setup()
{
   // generate 25kHz PWM pulse rate on Pin 3
   pinMode(pwmPin, OUTPUT);   // OCR2B sets duty cycle
   // Set up Fast PWM on Pin 3
   TCCR2A = 0x23;     // COM2B1, WGM21, WGM20 
   // Set prescaler  
   TCCR2B = 0x0A;   // WGM21, Prescaler = /8
   // Set TOP and initialize duty cycle to zero(0)
   OCR2A = 79;    // TOP DO NOT CHANGE, SETS PWM PULSE RATE
   OCR2B = 0;    // duty cycle for Pin 3 (0-79) generates 1 500nS pulse even when 0 :
   digitalWrite(2, HIGH);   // Starts reading
   Serial.begin(9600);
}
void loop()
{
 unsigned int x;
 // ramp up fan speed by increasing duty cycle every 200mS, takes 16 seconds
 //for(x = 0; x < 80; x++) {
 //  OCR2B = x;    // set duty cycle
 //  getRPMS();
 //  Serial.println(x);
 //  delay(200);
 //}
 while (Serial.available() == 0);
 int val = Serial.parseInt();
 if (val > 0 && val < 80) {
     OCR2B = val;
 }
}
char getRPMS() {
 time = pulseIn(2, HIGH);
 rpm = (1000000 * 60) / (time * 4);
 stringRPM = String(rpm);
 if (stringRPM.length() < 5) {
   Serial.println(rpm, DEC);
 }
}

Wiring: