You have to properly encode the video here is detail explanation with HD profile of what your mpeg4 video output should look like in *.mp4 format. You don't need to convert it with any other A/V codec as mpeg4 containers is sufficient if done correctly. I always encode my videos for mobile device before I transfer it thats another reason it looks good.
Codec: X264 2 Pass
Resolution: 800x480
Bitrate: 128kbps
Crop: 5:3 (1.667)