It's impossible for your actual throughput to exceed the rate that your ADSL synchronizes at; not only that, but it's also impossible for your throughput to even reach the actual reported speed that you're synchronizing at, because of IP overhead. 4Mbps is theoretically 512KB/s, but because of overhead we don't usually see more than 425KB/s with an MTU of 1492 on a 4Mbps line... some people claim to reach speeds of up to 475KB/s - but the point is nobody ACTUALLY ever hits 512KB/s - I've never seen a speedtest result that exceeds 3.6Mbps on a 4Mbps line.
The most common reason why people think they're downloading faster than their line is caused by a bug/illusion in most browsers, where you choose to save a file, and then the browser actually starts a buffered download of the file while you're choosing where to save it. Once you've chosen your destination and you click SAVE, BANG suddenly you're pumping 800K/s for a few moments and are on 10% already? Well that's only because it was downloading the file while you were finding where to save it.
A similar thing happens sometimes if you have a file that's already downloaded a certain percentage, and then later resumed... sometimes it takes a while for the "estimated time remaining" or "download speed" to be accurately reflected, because the previous chunk of the file gets incorrectly accounted for.
Another reason why you might see reported speeds greater than your actual sync speed, is if the transfer is being compressed, but the actual uncompressed transfer size is what's being reported (this happens when you use encryption on rsync, for example)
Edit: ah, rtzouves beat me to it
Install a program like Netlimiter or Netmeter/DUMeter, and you'll see your actual throughput wont exceed, or even actually reach 4Mbps, if that's what you're syncing at.
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