int brightness = 0;    // LED brightness
int increment = 5;     // brightness increment

void setup()
{
  pinMode(4, OUTPUT);      // set PWM pin as output
  setPWMResolution(4, 10); // set PWM resolution to 10-bit (now values range in 0-1023)
}

void loop()
{
  // increment brightness for next loop iteration
  brightness = brightness + increment;  
  // reverse the direction of the fading
  // since 10-bit resolution is active,
  // possible values are in the range 0-1023
  if (brightness <= 0 || brightness >= 1023) {   
    increment = -increment;
  }  
  
  // keep brightness in the range
  brightness = constrain(brightness, 0, 1023);
  analogWrite(4, brightness); // set the PWM output 
  
  // wait for 20 milliseconds to see the dimming effect
  delay(20);  
}