Line Attenuation and SNR margin is marginal. I'm currently on 4Mbps having attenuation of 38dB but my SNR is 20dB I pay the price though as I only manage to achieve max speed of 3.5Mbps due to my line...
The more pressing issue is the SNR margin though. You're currently capped at 2Mbps and its 11.6dB. To have a stable connection without much disconnection, the minimum SNR margin you should have would be at least 16dB. If your line is further capped at 4Mbps, your SNR will drop around 5dB which leaves you a small margin of 6dB for 4Mbps. Having a SNR at 6dB will experience frequent disconnections and would not achieve maximum speed.

Not recommended to upgrade to 4Mbps. But if you're keen to do so, replacing the copper cables in your house might help. Or, you can try plugging it into another telephone socket, if you have more than 1 socket. I've experienced a different line attenuation at different socket in my house. Hope it helps

4km btw is too far for 4Mbps if i'm not mistaken

This post has been edited by Rayckl: Jul 10 2010, 10:15 PM

mine keep fluctuates around 300kbps and 3.5Mbps in this few days...