from machine import Pin
import time

time.sleep(0.1)

led1 = Pin(12, Pin.OUT)
led2 = Pin(13, Pin.OUT)
led3 = Pin(14, Pin.OUT)
led4 = Pin(15, Pin.OUT)

LEDS = [led1, led2, led3, led4]

while True: 
    for k in LEDS:
        k.on()
        time.sleep_ms(500)
        k.off()

    for i in range(len(LEDS)-2, 0, -1):
        LEDS[i].on()
        time.sleep_ms(500)
        LEDS[i].off()