Solution:
1. Confirm whether Xiaomi Smart Air Purifier 4 is powered on and running normally;
2. Confirm whether the Wifi is normal;
3. Please unplug the power cord, and then turn it on after 10 seconds (the power needs to be discharged for a while), try to reset the Wifi of Xiaomi Smart Air Purifier 4, and reconnect to the Wifi.
Does it help?