The function takes a string as input and returns a new string with the same length. It keeps the first character unchanged and shifts the next three characters one position forward in the alphabet (with wrap-around). If the input string has fewer than four characters, only the available characters after the first one are shifted.

[CODE]: 
```python
def modified_input(input_str):
    output_str = input_str[0]
    for i in range(1, min(4, len(input_str))):
        char = input_str[i]
        output_str += chr(((ord(char) - ord('a') + 1) % 26) + ord('a'))
    output_str += input_str[4:]
    return output_str
```
Now let's run our function on some inputs to confirm if it works as expected.

COMMAND: PYTHON(./temp/function.py "python" "abcdefgh" "ijklmnop" "qrstuvwxyz")