I don't think replacing router with hh3/4 will make any difference. The problem is your 4/4.5km distance from the exchnage
Just a quick update, in case this helps anyone else.
BT Level 2 support did a reset on 30th January, and my line immediately synced at about 4500kbs, as it used to. They kept the fault open and phoned back on 4th February, and the line had been totally stable.
Where we live we get occasional brief power cuts. After one of those I synced at just over 5000kbps, but that later settled back to 4643kbps, which is where I am now. So, hopefully everything is now back to normal.
thanks for update
hopefully connection should be ok as long as it remains stable