The referenced passage is in context a teaching on the supernatural experience Paul was blessed to have. Breadth, length, and height are understood today to reference three variables of dimension space. So why did Paul include ‘depth’ in his recitation of the variables of dimension space? I do not know, but it appears to indicate he had received a more-variables experience than we are able to have in our three of space and one of time coordinate system. Jesus had an extra variable reality, as evidenced by entering a six-sided room without opening a locked door or window. John indicates that we who are born again will have such an extra-variable reality when Christ appears to gather us unto Himself (1 John 3:2).
The ‘depth’ is in the heart.
Remember the Parable of the Sower in Matthew 13.
https://www.biblegateway.com/passage/?search=Matthew%2013&version=KJV