The function swaps the third and fourth characters in the input string if and only if the third character is 'e' and the fourth character is 'l'. It returns the input string unchanged otherwise.

[CODE]:
```python
def approximate_function(x: str) -> str:
    if len(x) >= 4 and x[2] == 'e' and x[3] == 'l':
        return x[:2] + x[3] + x[2] + x[4:]
    else:
        return x
```