def hmsm_to_days(hour=0, min=0, sec=0, micro=0):
    """
    Convert hours, minutes, seconds, and microseconds to fractional days.
    
    Parameters
    ----------
    hour : int, optional
        Hour number. Defaults to 0.
    
    min : int, optional
        Minute number. Defaults to 0.
    
    sec : int, optional
        Second number. Defaults to 0.
    
    micro : int, optional
        Microsecond number. Defaults to 0.
        
    Returns
    -------
    days : float
        Fractional days.
        
    Examples
    --------
    >>> hmsm_to_days(hour=6)
    0.25
    
    """
    days = sec + micro / 1000000.0
    days = min + days / 60.0
    days = hour + days / 60.0
    return days / 24.0
