如何排列4个字符的组合，而不是重复？

def generate_combinations(combination, length, available_chars, result):
    if length == 4:
        result.append(combination)
    else:
        for char in available_chars:
            new_combination = combination + char
            new_available_chars = available_chars.copy()
            new_available_chars.remove(char)
            generate_combinations(new_combination, length + 1, new_available_chars, result)
            new_available_chars.append(char)

result = []
generate_combinations("", 0, ["A", "B", "C", "D"], result)
for combination in result:
    print(combination)

ABCD
ABDC
ACBD
ACDB
ADBC
ADCB
BACD
BADC
BCAD
BCDA
BDAC
BDCA
CABD
CADB
CBAD
CBDA
CDAB
CDBA
DABC
DACB
DBAC
DBCA
DCAB
DCBA