Angelina and her friends formed a club named the extremely cool club. they wanted to assign a unique 4-digit secret code number to each member of the club. they decided to use the digits 1, 3, 7, and 9 for their numbering system and each of these digits can appear only once in every secret code number (i. e. 1379 is a valid number, but 1133 is not a valid number). what is the maximum number of members who could join the club if everyone is to be assigned a unique secret code number?