Asus had included the Zenfone 4 in its plans to update older phones to Android Pie (9.0). However, this device has just been excluded from this schedule. The OEM claims that this will conserve the 4's system integrity and general function, however.
https://www.notebookcheck.net/Asus-arrests-development-for-its-Zenfone-4-Android-Pie-update.440354.0.html