the 1st gen appletv doesn't support the video spec that the iphone4 creates.
iphone4 creates a 720p 30fps file, and the 1st gen appletv only support 720p at 25fps.
you will need to convert the video to make it 1st gen appletv compatible.
you can convert it to 540p at 30fps (advisable option), or keep it at 720p and change the framerate to 25fps (not advisable).
in itunes, highlight the video, and then in the advanced menu, choose "create ipad or appletv version".