def los_id_4(df):
    """Line-of-sight id 4

    Helper predicate for sample validation

    Parameters
    ----------
    df : pandas.DataFrame
        DataFrame containing the measurements for this window

    Returns
    -------
    bool
        True if `df` contains los_ids 0, 1, 2, and 3
    """
    return set(df.los_id) == set([0, 1, 2, 3])
