Brute Force with 4 digits

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
NUMBERS = [str(number) for number in numbers]
letters = "abcdefghijklmnopqrstuvwxyz"
LETTERS = letters.upper()
possible = list()
for i in range(len(NUMBERS)):
    possible.append(NUMBERS[i])
for i in range(len(letters)):
    possible.append(letters[i])
for i in range(len(LETTERS)):
    possible.append(LETTERS[i])
print(len(possible))

from timeit import default_timer as timer
counter = 0
start = timer()
for a in possible:
    for b in possible:
        for c in possible:
            for d in possible:
                counter += 1
                print(str(counter) + ":" +  str(a + b + c + d))

end = timer()
elapsed = end - start
print("Elapsed time: " + str(elapsed) + " seconds.")