For the time being, it is understood as all the combination methods of letters in a string, as follows, a violent and ugly exhaustive method. . . I would like to ask if there is any better method. I have tried several methods in itertools, but none of them meet what I want. Thank you!

base='ATCG'
list=[]
for i in base:
    for j in base:
        for k in base:
            for m in base:
                for l in base:
                    for n in base:
                        seq=i+j+k+m+l+n
                        list.append(seq)
print(len(set(list)))
4096