The actions you can perform in MPEG4 Modifier are changing aspect ratio, editing user data, removing or adding packed bitstream, changing interlaced field order, finding out information about the video, such as the amount of I/P/S/B-VOPs used, if QPel/GMC were used or not and so on.
Nevertheless, the movie must be found in an AVI container.
A thing you need to be aware of is that aspect ratio modification works by changing a flag in the VOL headers, which is ignored by most decoders.
To sum up, MPEG4 Modifier is easy to use and reliable; it needs a very low amount of resources and it helps you save time by not using any conversion methods.
Avoiding re-encoding saves time and resources, and this is the main advantage of MPEG4 Modifier, compared to other editors.