# Mill Invisible Smart WiFi Panel Heater 400W Gen. 4 (PA400WIFI4)

Mill's PA400WIFI4 is a compact 400 W panel heater with direct Matter over Wi-Fi and no required hub. Its fit depends on accepting a strict choice between Matter control and Mill's own app features.

Canonical page: https://matterhome.io/devices/mill-invisible-smart-wifi-panel-heater-400w-gen4
Markdown page: https://matterhome.io/devices/mill-invisible-smart-wifi-panel-heater-400w-gen4.md
Author: Matterhome Editorial Team
Author profile: https://matterhome.io/authors/matterhome-editorial-team
Editorial lead: JC Martinez
Editor profile: https://matterhome.io/authors/jc-martinez

## Device Facts

- Brand: Mill
- Model: Invisible Smart WiFi Panel Heater 400W Gen. 4 (PA400WIFI4)
- Product type: Matter-over-Wi-Fi panel heater
- Category: Thermostats
- Connection: Matter over Wi-Fi
- Matter status: Mill documents current Matter-over-Wi-Fi activation for the exact PA400WIFI4 Gen 4 heater; Matter mode replaces the Mill app and Mill Cloud control path rather than running alongside it.
- Launch date: 2025-08
- Thread border router required: No
- Brand hub required: No
- Typical price range: NOK 1,449 at checked Norwegian retailer
- Markets: Europe
- Review status: Compatibility guide
- Product image: https://assets.matterhome.io/content/devices/mill-invisible-smart-wifi-panel-heater-400w-gen4/product.acdf143f3c78feb9c024/source.webp
- Product image alt text: White Mill Invisible 400W Gen 4 panel heater beside its retail box
- Product image source: https://millnorway.com/wp-content/uploads/2022/06/PA400WIFI4_product_photo_ean_7090019825308_image_6.jpg

## Ecosystem Support

- Apple Home: Supported
- Google Home: Supported
- Alexa: Supported
- SmartThings: Not listed
- Home Assistant: Not listed

## Best For

- Small bedrooms, home offices, entry areas, and other rooms of about 4 to 6 square metres
- Apple Home, Google Home, or Alexa households that want temperature control without a Thread border router
- Buyers who prefer their main smart-home platform over Mill's schedules and energy dashboard

## Skip If

- The room needs more than 400 W of heat after a proper heat-loss calculation
- Mill app schedules, power monitoring, and Matter control must remain available together
- SmartThings or Home Assistant support must be documented for the exact model before purchase
- Local electrical or bathroom rules do not allow this plug, mounting position, or installation method

## Setup Notes

- Verify model PA400WIFI4 and EAN 7090019825308; this guide covers the white 400 W Norwegian-market version rated for 220 to 240 V.
- Provide a compatible Matter controller on 2.4 GHz Wi-Fi; a Thread border router and Mill hub are not required.
- If the heater is already connected to the Mill app, reset it before starting Matter activation.
- In the Mill app, choose Settings and Activate Matter, follow the prompts, then use the emailed QR code or PIN in Apple Home, Google Home, or Alexa and press the heater's Wi-Fi button when instructed.

## Known Limitations

- Activating Matter removes the Mill Cloud path, so app schedules, power monitoring, and other app controls should not be assumed to remain available.
- Mill does not publish an exact cross-platform matrix for temperature, mode, schedule, energy, or automation controls.
- IPX4 and Mill's zone 2 bathroom claim do not replace local electrical, mounting, or bathroom-zone requirements.
- Matterhome has not tested heating performance, surface temperature, energy use, Wi-Fi range, pairing, automations, or multi-admin behavior.

## Pros

- Uses direct Matter over 2.4 GHz Wi-Fi without a Mill hub or Thread border router
- Sizes a built-in 400 W heater for compact rooms of about 4 to 6 square metres
- Includes a wall bracket, open-window detection, local locks, and an IPX4 rating
- Mill documents Apple Home, Google Home, and Alexa for the exact Gen 4 range

## Cons

- Matter and the Mill app cannot control the heater at the same time
- Matter activation still requires the Mill app, a Mill account, and access to emailed setup credentials
- The 400 W output is too small for many living rooms and open-plan spaces
- Mill does not list exact SmartThings or Home Assistant support for this model

## Sources

- https://millnorway.com/product/mill-invisible-wifi-panel-heater-400w/
- https://millnorway.com/mill-launches-smart-wifi-generation-4-panel-heaters/
- https://intercom-help.eu/mill-norway/en/articles/286873-how-do-i-activate-matter
- https://intercom-help.eu/mill-norway/en/articles/286875-can-i-use-matter-and-the-mill-app-at-the-same-time
- https://intercom-help.eu/mill-norway/en/articles/286874-how-do-i-remove-matter
- https://www.clasohlson.com/no/Mill-Invisible-Smart-wifi-ovn-Gen.-4%2C-hvit%2C-400-W/p/46-1376
- https://millnorway.com/wp-content/uploads/2022/06/PA400WIFI4_product_photo_ean_7090019825308_image_6.jpg

Mill's 400 W Invisible Gen 4 is a slim electric panel heater for a small room, not a whole-home heating system. The exact white `PA400WIFI4` is rated for about 4 to 6 square metres and was listed at NOK 1,449 by Clas Ohlson Norway when checked.

It connects directly through Matter over 2.4 GHz Wi-Fi, without a Mill hub or Thread border router. The important tradeoff is unusually sharp: Mill says Matter and its own app cannot control the heater at the same time. Matterhome has not tested the heater or its integrations.

## Choose the control path before setup

Mill's [Matter activation guide](https://intercom-help.eu/mill-norway/en/articles/286873-how-do-i-activate-matter) starts in the Mill app. Create or sign in to a Mill account, open Settings, choose Activate Matter, and follow the prompts for the heater. Mill then emails a Matter QR code and numeric PIN for Apple Home, Google Home, or Alexa. The heater's Wi-Fi button completes the commissioning step when the chosen ecosystem asks for it.

That use of the Mill app is temporary if Matter is the destination. Mill's separate [coexistence answer](https://intercom-help.eu/mill-norway/en/articles/286875-can-i-use-matter-and-the-mill-app-at-the-same-time) says the two control paths cannot be used together. Once Matter is active, the heater can no longer connect to Mill Cloud.

This choice matters because the [product page](https://millnorway.com/product/mill-invisible-wifi-panel-heater-400w/) also advertises weekly schedules, power monitoring, predictive heating, and other app features. Do not assume those Mill app controls remain available in Matter mode. Mill does not publish a platform-by-platform table showing exactly which temperature, mode, energy, and schedule functions Matter exposes.

Apple Home, Google Home, and Alexa are named for this Gen 4 range. SmartThings and Home Assistant are not. They may support the relevant Matter device type, but that generic possibility is weaker than exact-model documentation. Verify the current integration before buying specifically for either platform.

## The 400 W size is intentionally narrow

Model identity is easy to blur because Mill sells several Invisible heaters with similar names and different outputs. This guide covers `PA400WIFI4`, EAN `7090019825308`, the white 400 W Gen 4 version sold in Norway and rated for 220 to 240 V. The official range also includes larger models, but their room coverage, dimensions, price, and availability differ.

Mill rates this one for a room of roughly 4 to 6 square metres. The cabinet measures 49.2 by 38.4 by 8.6 centimetres, and a 1.3 metre power cord exits on the right. A wall bracket and screws are included. Open-window detection, child and commercial locks, a claimed maximum surface temperature of 71 degrees Celsius, and an IPX4 enclosure are part of the physical product rather than Matter features.

Mill says the heater is approved for bathroom zone 2. That statement does not overrule local electrical rules, socket placement requirements, or mounting clearances. Confirm the installation with a qualified person when the heater will be near water. A heat-loss calculation is also more useful than a floor-area estimate in a poorly insulated room.

## Commission it before mounting permanently

The heater needs 2.4 GHz Wi-Fi. Bluetooth 5.2 assists setup, but it is not the ongoing Matter transport. A compatible Matter controller on the same home network is required; a Thread border router is not.

If the heater is already registered in the Mill app, reset it before activating Matter. Commission it near the intended access point and prove the daily controls before fixing the bracket in its final position. Check temperature changes, heating state, household access, automations, and recovery after a short power interruption. Also confirm that giving up the Mill app's schedules and power view is acceptable.

Returning to Mill Cloud requires removing Matter and performing Mill's documented master reset. The [removal guide](https://intercom-help.eu/mill-norway/en/articles/286874-how-do-i-remove-matter) uses the local gear button to reach `rA`, followed by `YES`. Keep the printed label and emailed Matter credentials somewhere recoverable before installation.

## Buy it for one compact room

The [Clas Ohlson Norway listing](https://www.clasohlson.com/no/Mill-Invisible-Smart-wifi-ovn-Gen.-4%2C-hvit%2C-400-W/p/46-1376) offered the exact model with normal shipping when checked. That makes it a current product rather than a certification-only listing. Buyers elsewhere in Europe should still verify the exact model number, plug, voltage, warranty route, and local stock.

Choose `PA400WIFI4` when a small room genuinely needs about 400 W and Apple Home, Google Home, or Alexa will be the main control surface. Direct Matter over Wi-Fi is useful here because the heater needs no separate bridge.

Skip it when the room needs more heat, when app energy history and weekly schedules are essential, or when Matter and the Mill app must work side by side. The strongest purchase case is a compact room, a documented ecosystem, and a buyer comfortable committing to one software path.