Just to make sure:
Even if it's 100Hz natively (and indeed, there are plenty of sets that are), it's only 100Hz on the OUTPUT. The input will still be limited to 60Hz. There are unofficial tricks to get 100/120Hz even on the input, but they are not guaranteed to work on every set.
http://www.blurbusters.com/overclock/120hz-pc-to-tv/
Furthermore, like said, the 400Hz is purely marketing. The panel tech (IPS, VA) is limited to around 120Hz. There is nothing on the market that is even 200Hz natively, as far as I can tell. All the extra is just inflated numbers to indicate extra features, like motion interpolation. The only thing that would even theoretically be able to reach 400Hz (in visual sense), would be LED backlight flickering (PWM). But most probably there's nothing reaching anything above 150Hz.
For example, my set advertises 700Hz. But it's 100Hz natively, and I don't think even the motion interpolation reaches above 60Hz, if even that (furthermore, Philips quality in general is quite poor). And bear in mind, TV broadcasts are 25Hz in PAL, 30Hz in NTSC, and movies are 24Hz. When the motion interpolation works in double, it would still be a noticeable difference (if done right). From what I've seen, Sony has quite good motion interpolation in their upper tier.