How to move a box [ rectangle as square] up and down in pygame?

I am trying to move a box up and down in pygame.
I can move the box left using key a and right using key d.
How can I move it up and down?
My code:

import sys
import pygame as pg


def main():
    screen = pg.display.set_mode((640, 480))
    clock = pg.time.Clock()
    rect = pg.Rect(300, 220, 20, 20)
    velocity = (0, 0)
    done = False

    while not done:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                done = True

        keys = pg.key.get_pressed()
        if keys[pg.K_a]:  #to move left
            rect.x -= 4
        if keys[pg.K_d]: #to move right
            rect.x += 4


        screen.fill((40, 40, 40))
        pg.draw.rect(screen, (150, 200, 20), rect)

        pg.display.flip()
        clock.tick(30)


if __name__ == '__main__':
    pg.init()
    main()
    pg.quit()
    sys.exit()