From Korea to VietNam, I have to transit in 4hrs in Shenzhen (Seoul - Shenzhen - Hanoi)

If you are flying on Shenzhen Airlines your entire journey, you will be fine, but if not, you are advised to apply and obtain a Chinese transit visa in advance. The problem is in the fact that Shenzhen although allowing 24 hour visa free transit, Timatic has not yet been updated for this fact. However, Shenzhen Airlines personnel knows well about this detail and they will allow you to board your plane in Seoul without any problem.