Matt what I should have told you to do is take the faceplate off and try the test socket, as this bypasses all the extenstion cabling stuff.
It should be easy, 2 screws come off and the front comes off like a lego piece, behind that you see another socket like the socket on the faceplate, plugin the router there as normal and see if the stats improve.
If they do then you need to do a more permenant fix in removing the ringwire.
The 4mbit you getting isnt a big shock considering your noise margin.
Welcome to the world of adsl where its entirely possible to only burst to a fraction of the max speed.