------
## 最终结论
```python
def max_freq_substring(s, maxLetters, minSize, maxSize):
    from collections import defaultdict

    substring_count = defaultdict(int)
    max_occurrences = 0
    
    # 遍历所有可能的子字符串
    for size in range(minSize, maxSize + 1):
        for i in range(len(s) - size + 1):
            substring = s[i:i + size]
            
            # 检查子字符串条件
            if len(set(substring)) <= maxLetters:
                substring_count[substring] += 1
                max_occurrences = max(max_occurrences, substring_count[substring])
    
    return max_occurrences

# Example usage:
# print(max_freq_substring("aababcaab", 2, 3, 4))  # Output: 2
# print(max_freq_substring("aaaa", 1, 3, 3))       # Output: 2
# print(max_freq_substring("aabcabcab", 2, 2, 3))  # Output: 3
# print(max_freq_substring("abcde", 2, 3, 3))      # Output: 0
```