Were there actually four-digit TN highways???
I have found a few old maps that show them.
This would be the first I've heard of this.
The first is an old map of Chattanooga. It shows what is now TN 153 as "4327".
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi1209.photobucket.com%2Falbums%2Fcc395%2FBrian5561%2FOldChatta-ChickDam.png&hash=02a4b7baab6fe2e7ac433b73cde37083faa6268d)
The second is an old Marion County general highway map. It shows multiple four-digit highways.
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi1209.photobucket.com%2Falbums%2Fcc395%2FBrian5561%2Foldmarionco1.png&hash=ace92b94c372bbcd37e7eaac03f4bcc916ee7445)
I'm guessing those are federal-aid secondary numbers for non-state highway portions of the FAS system.