What is the format and resolution of the video? Right click on the item in iTunes and seled Get Info.
It may be a 1080P video which is not compatible with the 4G. The 4G pod can play:
- H.264 video up to 720p, 30 frames per second, Main Profile level 3.1 with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats
- 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 per channel, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats
- Motion JPEG (M-JPEG) up to 35 Mbps, 1280 by 720 pixels, 30 frames per second, audio in ulaw, PCM stereo audio in .avi file format
Did you try using the iTunes option FIle>Create New Version>Create iPod version?
Also see:
Transferring or downloading an iTunes Digital Copy