The download speed of 4 to 8 refers to megabits per second, whereas 50 is referring to 50 kilobytes per second. There are 8 bits to a byte, so 500kbps = 4mbps.
This is what you'd expect if your connection was running at optimum pace. With torrents though, there are many overheads (especially if you don't have Port Forwarding) which use up your bandwidth as well.
The upload speed is simply how quick you can transfer from your PC to somewhere else, ie when sharing a torrent.
i used to get over 250kb/s on a 2MB connection....so 50 seems a little low to me....
Torrent speeds depend on so many factors; the most important being number of seeds, number of peers, overall swarm speed and your own connection speed. It's very rare (in my experience) that the limiting factor for a torrent download will be your own connection speed, it's far more likely to be one of the other factors I mentioned.
It is very rare that you will exceed even 1mb with a torrent, there is just so many variables.