I uploaded a 1280x720 AVI file encoded at 8000 kb/s to youtube. It takes a long time to upload. I uploaded another 1280x720 AVI file encoded at 4000 kb/s, and I don't see any difference in quality. Is there a difference in quality or not? Is 8000 kb/s too much? What about 4000 kb/s? What's the best bitrate to use?
Youtube performs transcoding to its own formats to optimize for bandwidth, so as long as the quality is better than those, you would see no difference in quality.
I believe youtube's bitrate for 720p videos is around 2Mbps, so at 4Mbps you should be fine. Since they do there own encoding it's preferable you upload it at the original bitrate since the time you take to convert it's probably the same you spend uploading.
8000 kb/s too much?
8000 kb/s would mean that the client would require a similar bandwidth to stream it without any problems. Obviously, that's a bit too high. There's no "best" bitrate as such - I often use a bitrate of ~ 600 Kb/s so that a person with a 512k connection can stream it pretty much instantaneously.