The function f(x) takes a string as input and applies a simple shift cipher to each alphabetic character in the input string. The shift value used is constant. Non-alphabetic characters are not changed.

[CODE]:
```python
def f(x):
    shift = 4  # Choose the shift value or experiment with different values
    output = ""
    for char in x:
        if char.isalpha():
            if char.islower():
                char = chr((ord(char) - ord('a') + shift) % 26 + ord('a'))
            else:
                char = chr((ord(char) - ord('A') + shift) % 26 + ord('A'))
        output += char
    return output
```

You can adjust the value of the `shift` variable to experiment with different shift values and see how it affects the output.