There were a few reasons why the main lead of 'A Smile Is Alluring' hadn't changed at all in the past four years: 1. The script itself has not changed significantly: The script of the TV series has not changed significantly in the past four years. The performances of the leading actors are based on the original script, so their image and performance have not been greatly affected. 2. The personal image of the actors has not changed: the performance style and image of the leading actors are relatively fixed. There has not been much change in the past four years, which helps them to better shape the characters and maintain the unity of the image. 3. No change in filming conditions: The filming conditions of the TV series may not change much in the next four years. For example, the filming location, lighting, clothing, makeup, etc. These conditions will help the main actors maintain their original image and performance. 4. Self-management of actors: The main actors may pay attention to self-management and maintenance to maintain their health and image within four years. The reason why the lead actors didn't change in the past four years was probably due to many reasons, including the script itself, the actors 'personal image, the filming conditions, and their self-management.