def list_strings(string1='', string2='', string3='', string4=''):
    """
    Put strings in a list.

    Parameters
    ----------
    string1 : string
    string2 : string
    string3 : string
    string4 : string

    Returns
    -------
    string_list : list of strings

    Examples
    --------
    >>> from mindboggle.guts.utilities import list_strings
    >>> string1 = 'a b c'
    >>> string2 = 'd e f'
    >>> string3 = ''
    >>> string4 = 'j k l'
    >>> string_list = list_strings(string1, string2, string3, string4)
    >>> string_list
    ['a b c', 'd e f', 'j k l']

    """

    string_list = []
    if string1 and isinstance(string1, str):
        string_list.append(string1)
    if string2 and isinstance(string1, str):
        string_list.append(string2)
    if string3 and isinstance(string1, str):
        string_list.append(string3)
    if string4 and isinstance(string1, str):
        string_list.append(string4)

    return string_list