primitive password cracker

[Bischoop]

I was practising for educational purpose with simply password cracker.
I know that I could use itertool but in my case when I'm learning I
would miss facing problems and learning on them and indeed I've met one
which is not giving me a peace.

What I want to learn is if I need get for example four combinations, so
how to get in a loop first letter 'a',then another step'a' and again 'a'
and 'a', to have 'aaaa' later on'abaa' etc.

So I wrote that:
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import string
passe = 'zulu'
mylist = []
#letters = string.ascii_lowercase
letters = ['a','b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'r', 's', 't', 'u', 'w', 'q', 'y', 'z']
mineset= set()
for a in letters:
    for b in letters:
        for c in letters:
            for d in letters:
                s = a + b + c + b
                mylist.append(s)
mineset=set(mylist)

k = sorted(mineset)
print(k)
for i in k:
    if i == passe:
        print('got it: ', i )
print(passe in k)
--------------------------------------
It works in someway but the problems are:
Not all combination are made, I change password to: 'pass' and that combination is not in results.
Another thing is, I had to made a set from list because combinations
were repeated.
And finally, I was struglling with making it in without creating four
loops for four letters, something like that:

To try to solve those I went with that approach:

-------------
letters = ['a','b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'r', 's', 't', 'u', 'w', 'q', 'y', 'z']
password = 'pass'
combin = ''
lista=[]

for x in range(4):
    for y in letters:
        combin +=y
        lista.append(combin)
    combin=''
mineset=set(lista)
print(mineset)
for i in lista:
    if i == password:
        print('pass:', i)
-------------------
But the results are disspainting:
{'abc', 'a', 'ab', 'abcd', 'abcde'}

I was seating on it for a long day but can't even closely achieve
similar effect to 4 loops in previous code.

--
Thanks