def solve(s):
    """
    Count all the uppercase letters, lowercase, numbers and special characters in a string.
    :param s: a string input.
    :return: a list of ints detailing the count of uppercase letters, lowercase, numbers and special characters,
             as follows.
    """
    lower = upper = num = char = 0
    for i in s:
        if i.isupper():
            upper += 1
        elif i.islower():
            lower += 1
        elif i.isdigit():
            num += 1
        else:
            char += 1
    return [upper, lower, num, char]