Mill Invisible 400W Gen 4

A small heater worth buying for the space it saves

Mill's Invisible 400W Gen 4 suits a tiny room where a portable heater would get in the way. The rounded white steel panel sits on the wall, leaving the floor clear and avoiding the industrial look of exposed fins. At 49.2 cm wide, it is appreciably smaller than many wall heaters. That compact shape is the reason to consider this particular model.

Clas Ohlson Norway listed the exact PA400WIFI4 at NOK 1,449 including VAT on September 10, 2026. Delivery cost depends on the method and destination. That buys the heater and wall bracket, with a 1.3-metre power lead exiting on the right. Check the socket position before letting the neat product photo decide where it should go.

Four hundred watts is a real limit

Mill recommends this size for roughly 4–6 m². Treat that as a sizing guide: insulation, outside temperature and heat loss still determine whether the room gets warm enough. A small panel that fits perfectly but cannot keep up with a cold room is a bad purchase.

The same Norwegian retailer listed the 700 W Gen 4, PA700WIFI4, at NOK 1,849 including VAT. The extra NOK 400 buys more heating capacity in a 65 cm-wide panel. If the wall has space and the room needs more heat, spending that money on the larger model makes more sense than hoping clever controls will stretch 400 W further.

Smart scheduling can reduce unnecessary heating, but it does not guarantee a particular bill reduction. We have not measured this heater's temperature stability, warm-up time or energy savings. Mill's energy-management claims are not a reason to undersize it.

The controls involve an awkward choice

The heater can join Apple Home, Google Home or Alexa through Matter over Wi-Fi without a separate Mill hub. For someone already using one platform to manage the home, adjusting the heater there is a useful convenience.

The catch is that Matter mode and Mill's own app cannot run together. Mill's app offers weekly heating programs and energy information; activating Matter means choosing the receiving platform's controls and routines instead. Do not buy it expecting all the advertised app features to follow it into Apple Home. Matter activation also begins in Mill's app, so this is not an entirely app-free purchase.

Physical plus and minus buttons still provide a straightforward way to choose a temperature without Wi-Fi. That is a welcome everyday detail when a guest just wants the room warmer. The heater does not need a phone for that basic job.

A tidy fixture, with fewer reasons to upgrade

This model needs its wall mounting; it is not a portable panel to prop against a desk. Keep the required clearances in mind when measuring the space. Its IPX4 rating is useful for an appropriate bathroom installation, but the manual's placement requirements and local electrical rules still matter.

Before replacing an older Mill, check what it already supports. Mill's current Matter activation guide includes Gen 3 panel heaters as well as Gen 4. An existing heater that warms the room properly may already have the connection you want, without buying a new panel.

Choose this 400 W model when its compact dimensions solve a real problem and you are comfortable choosing one app system. Keep a working heater when the only attraction is a newer generation number, and size up when the room needs more warmth.