Welcome to Yorkie Talk. It would be unusual for a 4 month old to have collapsed trachea unless it is a genetic defect, or the pup suffered neck/throat trauma, or the pup had some sort of illness to damage the throat.
Has the vet examined your pup? I believe that CT is diagnosed through x-rays. Maybe someone will correct me.
Here is a link with videos demonstrating the difference between kennel cough, CT, and reverse sneezing (common in Yorkies):
http://www.yorkietalk.com/forums/die...d-trachea.html
Collapsed trachea can be managed successfully depending on the severity of the case: keeping the pet's weight down helps, keeping the pet from overheating, and keeping the pet from becoming overly excited or physically exerted.
Keep us posted on what you learn. Hoping your pup does not have CT.