动态规划可以解决
def longest_common_substring(s1, s2, min_length=4):
m = len(s1)
n = len(s2)
dp = [[0] * (n + 1) for _ in range(m + 1)]
max_length = 0
end_index = 0

for i in range(1, m + 1):
for j in range(1, n + 1):
if s1[i - 1] == s2[j - 1]:
dp[i][j] = dp[i - 1][j - 1] + 1
if dp[i][j] > max_length:
max_length = dp[i][j]
end_index = i
else:
dp[i][j] = 0

if max_length >= min_length:
return s1[end_index - max_length:end_index]
else:
return ""

A = "我今天特别开心啊，因为今天是个好日子，我中了 500 万彩票。"
B = "今天不是个好日子，因为邻居中了 500 万彩票，我今天不开心。"

print(longest_common_substring(A, B))