The Raspberry Pi 4 needs a fan
Quote:  the metal casing helps
  spread that heat around

It does, to a degree. But that is absolutely not what we are seeing in that IR picture. Shiny bare metal is a mirror at the wavelengths that thermal camera uses. So what you see on thermal image where wifi-module shield and CPU are is reflection of ambient (room) where this picture was taken. Try waving your hand around, you'll see it reflecting there too.To measure real metal surface temperature by IR, you have to paint that metal (ideally, with black matte paint), or apply similarly-textured sticker.