ah... no, but here's what I think the deal is (I could be wrong of course). My gen III sometimes had issues with playback of my larger bluray files, 1) it had not nearly as much memory, and it 2) I suspect it was doing software decoding. My gen IV is blazingly fast at everything - the scrubbing speed (finger fast forward and rewind) is amazing. I think this might be do to hardware decoding of H.264. So.... if they decided to only support hardware decode of H.264, and reject high res other encodings, it could explain it. Now, if you compare the specs (below), it doesn't say that (they seem to support the same formats), but it's my theory based on what I'm seeing.
http://www.apple.com/shop/buy-tv/apple-tv-3rd-gen
MPEG-4 video up to 2.5 Mbps, 640-by-480 pixels, 30 frames per second, Simple Profile with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats
http://www.apple.com/tv/specs/
MPEG-4 video up to 2.5 Mbps, 640 by 480 pixels, 30 frames per second, Simple Profile with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats