Just to clear things up a bit. mp4v is not really anything. It is used in VLC to represent MPEG-4 Part 2 video, but in a technical sense is nothing.
MPEG-4 is the general name for a set of specifications defined by ISO/IEC 14496. There are several parts to this specification. These are the relevant parts:
- Part 2 - Commonly known as MPEG-4 Video, is a video compression format.
- Part 3 - Commonly known as MPEG-4 Audio, is and audio compression format.
- Part 10 - Commonly known as H264 or MPEG-4 AVC, is also a video compression format.
- Part 14 - Commonly known as MP4, is a container format.
H.264 is typically considered to be the better for video compression. It contains several features that MPEG-4 video does not including those listed here.
The biggest difference you will typically see is file size. H.264 will typically be able to compress video at a much lower bitrate than the standard MPEG-4 codec. You should be able to get comparable quality video with much smaller file sizes.