the problem with HDR400 is that it isn't actually improving the
dynamic range HDR isn't
technically about colour improvements, although the uplift in colours has become synonymous with the term for marketing HDR displays.
HDR is
supposed to be about improving the dynamic range, i.e the contrast and difference between bright and dark areas of the image. you can only achieve that with some form of local dimming for the backlight on LCD displays, otherwise you are stuck with just the panel's native contrast ratio whatever that might be. The issue and gripe with HDR400 certified screens is that as a spec it doesn't require local dimming to even be used to achieve that badge, and i can't think of any screen which includes it as a result! Yes, HDR400 screens can accept an HDR10 input signal, yes they can reach a bit brighter than many screens (that might be more commonly 250 - 300 cd/m2), and yes it might help with colour and brightness mapping for HDR content. Yes it might therefore look better than if you didn't have that "HDR" support when you view HDR content... but in no way is it actually doing anything to improve the dynamic range of the image, which is really the name of the game.