Is it correct that 640x480 mp4 ends up more blurry than a swf? And is it a matter of size? Which leave a not so smart solution with double export size and then resizing on "upload". Or can someone do some magic?
Swf keeps the vector data so quality is not affected by resolution.
Mp4s work with pixels, it rasterizes your animation based on the stage resolution.
640x480 is a pretty low res by today's standards. To get a sharper image all you need to do is increase your stage size (ex. if you want a full 1080p video your stage size should be 1920x1080). Export it, and you'll instantly get a higher quality video.
Here are some good settings for high-quality export. Then if you want an Mp4 to take it into Media Encoder.