Add PM4 device class for Ecowitt (#152568)

homeassistant/components/ecowitt/sensor.py

@@ -253,6 +253,7 @@ ECOWITT_SENSORS_MAPPING: Final = {
     ),
     EcoWittSensorTypes.PM4: SensorEntityDescription(
         key="PM4",
+        device_class=SensorDeviceClass.PM4,
         native_unit_of_measurement=CONCENTRATION_MICROGRAMS_PER_CUBIC_METER,
         state_class=SensorStateClass.MEASUREMENT,
     ),