For capturing video to use as webcam there are little MPEG 4 video servers available that work well. There is one troubling problem with these however. When capturing video to MPEG 4 they size the video at 704 by 480. This creates an unnatural aspect ratio with respect to the original video resulting in distortion of the webcam image. Is this a necessary feature of MPEG 4 compression, or could they just as well have chosen 640 by 480 to preserve the 4 by 3 natural aspect ratio of the original video?
I guess if it's locked in the software then there is nothing you can do about it. Usually though when capturing you can choose different resolutions. It would be strange to be locked into 704x480 because this is more of a TV resolution, like when capturing for DVD. Some software can be strange though and I doubt it's an MPEG4 issue.