The fourth factor is something that humans needed to sustain life. Without having to rely on other facilities, such as computers. Auto electrical, telephone, etc., by four factors, this man cannot be broken.
Four factors is essential to human life. Without other amenities. Such as computers, cars, electricity, telephone, etc., by a factor of four, this man can not be without. When absent, then it may affect their lives.
The basic is the human things necessary to live. Without other amenities, such as a computer, automotive, electrical, telephone, etc. by a factor of four that humans cannot lack.