------
## 最终结论
```python
def encrypt(s):
    encrypted_chars = []
    for char in s:
        if char.isalpha():  # Check if the character is a letter
            new_char = chr((ord(char) - ord('a') + 4) % 26 + ord('a')) if char.islower() else chr((ord(char) - ord('A') + 4) % 26 + ord('A'))
            encrypted_chars.append(new_char)
    encrypted_string = ''.join(encrypted_chars)
    return encrypted_string
```