A bit of a toss-up really, isn't it. Some people on these boards would kill for a 4+ download on an 8 line, I suspect... and the result depends on so much - age of copper & connections to the xchange, contention, to name but two.
And (not first-hand knowledge this, but...) BT have been known to charge for a call-out with no faults found. OTOH, you have shown that you have a non-standard master socket, so you might be ok there.
If you are not in the habit of downloading lots of big files - i.e. the difference of a slightly faster line isn't really noticeable - you may decide to live with what you've got, until the next hike of speeds, which seem to come every few years. My connection with the same isp has gone from 1 to 2 to 4 to 8 nominal since about 2002. In reality I get a fairly steady 6-7 at the same distance from the exchange as you are.