bool ledState = false;

void setup() {
  pinMode(A0, INPUT);
  pinMode(6, OUTPUT);
  pinMode(4, INPUT);
}

void loop() {
  if (digitalRead(4) == HIGH) {
    ledState = !ledState;
    delay(200);
  }

  if (ledState) {
    int val = analogRead(A0);
    val = map(val, 0, 1023, 0, 255);
    analogWrite(6, val);
  }
  else {
    analogWrite(6, 0);
  } 
}