What format is the video? Only the following formats are supported. 3GPP formats (H.263), ASF, H.264/AVC, M4V, MPEG-4, VC-1, WMV 9. If the format is unsupported, you could try converting the file first.
There are other codecs for mp4 files than the two supported. Just because it is mp4 does not mean it will play.