const byte relaisPin  =  2;
const byte buttonPin  =  3;
const byte ledPin     = 13;

const int RELAY_ON = HIGH;
const int RELAY_OFF = LOW;


const unsigned long period = 14400000ul; // 4 hours
unsigned long lastActivation = 0;

bool relayIsActive = false;

void relayOn() {
  digitalWrite(relaisPin, RELAY_ON);
  digitalWrite(ledPin, HIGH);
  lastActivation = millis();  // do this if you want the pulse only 4h after the last forced activation
  relayIsActive = true;
}

void relayOff() {
  digitalWrite(relaisPin, RELAY_OFF);
  digitalWrite(ledPin, LOW);
  relayIsActive = false;
}

void setup() {
  Serial.begin(115200);
  pinMode(buttonPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);    digitalWrite(ledPin, LOW);
  pinMode(relaisPin, OUTPUT); digitalWrite(relaisPin, RELAY_OFF);
}

void loop() {
  // forced ON ?
  if (digitalRead(buttonPin) == LOW)   // LOW means pressed
    relayOn();
  else
    relayOff();

  // time to pulse ?
  if (!relayIsActive && (millis() - lastActivation >= period)) {
    // pulse !
    relayOn();
    delay (200);
    relayOff();
    lastActivation = millis();
  }

}