int buttonPin = 2;
int current = 0;
long lastEventMillis = 0;

void setup() {
    pinMode(6, OUTPUT);
    pinMode(7, OUTPUT);
    pinMode(8, OUTPUT);
    pinMode(9, OUTPUT);
    pinMode (buttonPin, INPUT);
}

void loop() {
    long currentMillis = millis();

    // This is only true once every 1 second
    if ((currentMillis - previousMillis) > 1000)
    {
        // Apply when last event toke place
        lastEventMillis = currentMillis;

        update();
    }

    // When the button is pressed
    if (digitalRead(buttonPin) == HIGH)
    {
        // Apply when last event toke place
        lastEventMillis = currentMillis;

        // Reset current position
        current = 0;
        update();
    }
}

// Updates the position of the LEDs in order
void update() {
    // Turn all LEDs off
    digitalWrite(6, LOW);
    digitalWrite(7, LOW);
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);

    // Enable the LED that matches the current position
    digitalWrite ((current % 4) + 6, LOW);

    // Update the current position
    current = current + 1;
}