I have already told the probable reason for ur connection to act like 2mbps when it should have been 4mbps. It because of ur line stats. Even with the new pic, its pretty clear that line attenuation is 44 which should be around 20 to 22 and SRN margin is at 16 which should be 28 or over especially, if you want to have a 4mbps connection.
I can bet that your still be getting 2mbps speeds even though after the restart, the sync rate has gone back to 4mbps right? Thats what happen when ur line gets weak (resulting in higher attenuation values) or has distortion (resulting in lower SNR values) due to joints, cuts, rust in the line.
When this happens you start to get frequent disconnections cause your modem is trying to sync according to your subscribed data rate (which is 4mbps in ur case) but the line is not supporting it... so after some disconnection the modem syncs in with data rate which ur line supporting in it current weaker state. and that's how ur getting 2mbps speeds whether modem is syncing at ur subscribed data rate or lower.
As I understand from ur post , you have two PTCL lines. one is dead and other one is used with LDN. You try and fix your dead line and subscribe 4mbps PTCL connection. Go to ur relvent exchange the be authoritative about ur problem, after all, your paying for it with all the profits to the ISP and taxes to the govt. know the names of the personal and their designations whom are dealing with at that exchange.
As for LDN line, call ur ISP and get it fixed. But let me suggest that its always better if u opt for PTCL DSL if you have PTCL line. Having PTCL line and LDN DSL (since LDN does not have a its own line infrastructure), will always put you in between two rival companies specially, if one (LDN) is dependent on the other (PTCL) for the resolution of technical issues such as your. If it doesn't go well, then it will be YOU, suffering.
Besides a perfect 2or4mbps PTCL connection is far better then a perfect 2or4mbps LDN connection overall in my experience and IMO.
Here are my line stats for my 4mbps PTCL DSL
As you can see the SN margin (circled in red) and attenuation value (circled in blue), are perfect for my line. I got this new line installed over one and half month ago and was facing frequent disconnections due to higher attn from day one. values which crossing 30. I complained on help line, some guys came, nothing really happened. I went to my exchange, asked the fix it and threatened them to not pay the a single penny in the coming bill but in polite but authoritative manner. They sent same guys, they did something. Now the attn value is fixed on 21.5 for over a month and not a single disconnection, no matter how many times I restart the modem since then and my line stays perfect unless there's some back-end problem and I get 470KB/s down with ~85KB/s up speeds on torrents 99% of the time. It hits 500+KB/s on IDM.