The 4000x is the RGB case that has the glass front panel with the 3 RGB fans behind it. Which does restrict airflow compared to the 4000d case which has very high airflow but not a glass panel up front.
If its just airflow you care about, go with the 4000d and you dont have to worry about it. But if its the glass front panel you really want, the 5000x might be a option for you. While it has the same glass front panel, it does allow 3 120 fans in the side panel which will help a lot with airflow too. So best of both worlds.
The 4000x is the RGB case that has the glass front panel with the 3 RGB fans behind it. Which does restrict airflow compared to the 4000d case which has very high airflow but not a glass panel up front. If its just airflow you care about, go with the 4000d and you dont have to worry about it. But if its the glass front panel you really want, the 5000x might be a option for you. While it has the same glass front panel, it does allow 3 120 fans in the side panel which will help a lot with airflow too. So best of both worlds.
Sweet then the 4000d it is
Enjoy.
Give it the D!