import random


def random_string(length=4):
    """Generate a random string."""
    random_str = ""
    for i in range(length):
        random_integer = random.randint(97, 97 + 26 - 1)
        flip_bit = random.randint(0, 1)
        random_integer = random_integer - 32 if flip_bit == 1 else random_integer
        random_str += chr(random_integer)
    return random_str