this is because.
i) your modem is not fully synced to 4096/1024 profile rate..this is used for 4M data transfer.
ii) your line or pair seems to be faulty with very low parameters.
Get hold of the line man ask him to fix your pair from cabinet by refreshing the pairs or by changing the pair and shifting it to a better one.
try finding any joints in your line. If there are any then replace your wire from DP/pole till your modem. And that is done by lineman.
PTCL is PTCL it do anything with its users sometimes 2MB user get 4MB probably 4Mb user get 2MB or 1 MB.
Ask your lineman to fix the distortion in your line an you will get the full speef of 4 mbps
ok thanks everyone.